An Introduction to Laser Technology and Its Applications

Home , Arthur Ashkin, Doppler cooling, Excimer laser, Gas laser, Hippolyte Fizeau, Laser cooling

2018–2019

SCIENCEResource Guide Northwest Pa. Collegiate Academy - Erie, PA

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INTRODUCTION ...... 4 Rainbows ...... 21 Total Internal Reflection ...... 22 SECTION I: LIGHT ...... 5 Fiber-Optics ...... 23 Introduction ...... 5 Mirrors and Lenses ...... 23 Properties of Light ...... 5 Plane Mirrors ...... 23 Concave and Convex Mirrors ...... 25 Waves ...... 5 Concave Mirrors ...... 25 Wave Properties ...... 5 Convex Mirrors ...... 26 Electromagnetic Waves ...... 6 Lenses ...... 27 The Speed of Light ...... 8 Converging Lenses ...... 28 The Electromagnetic Spectrum ...... 9 Diverging Lenses ...... 29 Antennas and Radio Waves ...... 10 Spherical and Chromatic Aberration . . .29 Transparent vs. Opaque Materials ...... 11 Wave Optics ...... 30 Visible Light and Color ...... 12 Huygens’ Principle ...... 30 The Visible Spectrum ...... 12 Diffraction ...... 30 Selective Absorption and Reflection ...... 13 Interference ...... 31 Selective Transmission ...... 14 Superposition ...... 31 Blue Skies and Red Sunsets ...... 14 Double Slit Interference ...... 32 Section I Summary ...... 16 Diffraction Gratings ...... 34

Thin-Film Interference ...... 34 Northwest Pa. Collegiate Academy - Erie, PA SECTION II: OPTICS ...... 17 Polarization ...... 35 Introduction ...... 17 Optical Instruments ...... 37 Geometric Optics ...... 17 Simple Magnifier ...... 37 Microscopes ...... 37 The Ray Approximation ...... 17 Telescopes ...... 38 Reflection ...... 17 The Law of Reflection ...... 17 Section II Summary ...... 39 Specular and Diffuse Reflection ...... 18 Refraction ...... 18 SECTION III: LASERS ...... 41 The Law of Refraction ...... 19 Introduction ...... 41 Mirages ...... 20 Dispersion ...... 21 Light Emission and Absorption . . . . . 41

2018–2019 Science2 Resource Guide The Atom ...... 41 Scientific Applications ...... 67 Quantization ...... 42 Laser Spectroscopy ...... 67 Photons ...... 42 Confocal Laser Scanning Microscopy . . . .67 The Bohr Model ...... 44 Manipulating Tiny Objects ...... 68 Atomic Excitation and Emission ...... 45 Optical Tweezers ...... 68 Emission Spectra ...... 47 Laser Cooling ...... 70 Incandescence ...... 48 Atomic Clocks ...... 71 Absorption Spectra ...... 50 Nuclear Fusion ...... 71 Fluorescence ...... 51 Phosphorescence ...... 52 Medical Applications ...... 73 Lamps ...... 53 Laser Surgery ...... 73 Incandescent Lamps ...... 53 Refractive Eye Surgery ...... 75 Fluorescent Lamps and CFLs ...... 53 Laser Dermatology ...... 76 Light-Emitting Diodes ...... 54 Laser Dentistry ...... 77 Lasers ...... 55 Industrial Applications ...... 78 Laser Operation ...... 55 Laser Drilling ...... 78 Stimulated Emission ...... 56 Laser Cutting ...... 78 Population Inversion ...... 57 Laser Beam Welding ...... 79 Energy Levels ...... 58 Laser Marking, Engraving, and Etching ...... 79 Optical Cavity ...... 59 Photolithography ...... 80 Properties of Laser Light ...... 59 Stereolithography ...... 80 Monochromatic ...... 60 Other Applications ...... 81 Directional ...... 60 Barcode Readers ...... 81 Coherent ...... 60 Laser Pointers ...... 81 Types of Lasers ...... 60 Laser Printers ...... 82 Gas Lasers ...... 60 Optical Discs: CDs, DVDs, and Others . . . .83 Solid-State Lasers ...... 61 Holography ...... 84 Semiconductor Diode Lasers ...... 62 Section IV Summary ...... 86 History of the Laser ...... 62 Northwest Pa. Collegiate Academy - Erie, PA 1900–50: Early Foundations ...... 62 CONCLUSION ...... 87 Early 1950s: Charles Townes and the Maser ...... 63 TIMELINE ...... 88 Late 1950s: Laser Conceived ...... 63 1960s: The Race to Build the Laser ...... 64 GLOSSARY ...... 90 1970s–Present ...... 64 Section III Summary ...... 65 NOTES ...... 94 BIBLIOGRAPHY ...... 95 SECTION IV: APPLICATIONS OF LASERS ...... 67 Introduction ...... 67

2018–2019 Science3 Resource Guide Introduction

The laser is among the most important inventions of the the phenomena of reflection, refraction, and diffraction. As twentieth century. Since its introduction in 1960, the we will see, lenses and mirrors can be configured to focus laser has made possible a countless number of scientific, light in particular ways, which will become important in medical, industrial, and commercial applications. Theodore understanding laser operation and applications. Maiman, the inventor of the first working laser, was quoted by The New York Times in 1964 as calling the laser “a Section III will provide an overview of the laser itself. solution seeking a problem.” Today it’s nearly impossible We will start by considering how light emission can be to go an entire day without using at least one object that explained using a quantum model of atomic excitation, and was either manufactured using lasers or that uses lasers in explore various mechanisms of light emission. By the end order to function. In this Science Resource Guide, we will of this section, you will be familiar with the most important explore the nature of light and optics, the basic operation properties of laser light, the main categories of lasers, and of a laser, and the various applications of lasers that have the history behind the development of the laser. transformed our lives over the course of more than half Finally, Section IV will provide an overview of major a century. Along the way, we will consider significant applications of lasers across several categories. Lasers are milestones in the development of the laser, as well as the an important tool in a variety of research fields, and they people who contributed to our understanding of both lasers have also made possible new surgical procedures and and light itself. You’ll also have opportunities to explore manufacturing techniques. Lasers are all around us, in laser light-related phenomena using online simulations and pointers, laser printers, and optical disc readers. By the end simple demonstrations with everyday materials. of this guide, you will be able to explain how lasers work Section I will cover the basic properties of light as an and the significant ways in which they contribute to our electromagnetic wave. In this section, we will discuss how daily lives. visible light represents a relatively small segment of the complete electromagnetic spectrum. We will also explore how the colors that we see are caused by interactions NOTE TO STUDENTS: You will notice as you read through between visible light and matter. the Resource Guide that some key terms and phrases are Northwest Pa. Collegiate Academy - Erie, PA boldfaced. These terms are included in the glossary at the In Section II, we will consider the field of optics, which end of the Resource Guide. concerns the behavior of light, the formation of images, and

2018–2019 Science4 Resource Guide Section I

Light

INTRODUCTION disturbs the water further out, which disturbs the water further out, and so on (Figure 1). Light is both a familiar and fascinating phenomenon. Although a laser beam and a flashlight beam have very Waves can be classified as transverse or longitudinal. A different properties, they are still governed by the same transverse wave is made up of oscillations that are fundamental principles that describe light. In this section, perpendicular to the direction that the wave travels. As we will set the stage for our discussion of the laser by we will see, light is an example of a transverse wave. In covering some of the most fundamental properties of light contrast, for a longitudinal wave, the oscillations are and color. parallel to the direction that the wave travels. Sound is an example of a longitudinal wave. Regardless of their classification, all waves are caused by a vibration of some PROPERTIES OF LIGHT kind. For example, sound waves originate from vibrations of matter, such as a person’s vocal chords or a guitar WAVES string, which initiate a sequence of collisions between air Many phenomena in the known universe are described by molecules that eventually reaches your eardrum. the general mathematical concept of waves. For example, the beam of light produced by a laser, the sound generated Waves transport energy but not mass. That is, waves do not by a thunderclap, and the ripples of water in a pond are carry matter along with them from one point to another as all waves. A wave is the motion of a disturbance between they travel. Think about a floating buoy bobbing up and two points. You’ve likely observed how dropping a small down in the ocean as waves pass. Although the height object, such as a pebble, into a pond causes ripples of of the buoy changes with time, on average its position water to spread outward from where the object lands. The along the surface of the water does not change. Likewise, pebble disturbs the water immediately surrounding it, which individual particles that make up the wave are not moving along with it. Likewise, a sound wave is not carried by individual air molecules moving all the way from the source FIGURE 1 to your ear. Although the individual molecules vibrate back Northwest Pa. Collegiate Academy - Erie, PA and forth along the direction of the wave, they exhibit no net motion toward your ear. WAVE PROPERTIES Although different types of waves have different physical properties, there are general characteristics that we can use to describe any wave. The amplitude of a wave is the maximum displacement from the equilibrium position. For a water wave or a wave on a string, the amplitude is the height of the wave compared to its resting position. For electromagnetic waves, the amplitude refers to the maximum magnitude of the electric and magnetic fields. In A disturbance event, such as a dropped pebble, general, the amplitude of a wave is related to how much causes waves on the surface of a still pond. energy it carries. The wavelength of a wave is defined

휆

2018–2019 Science5 Resource Guide FIGURE 2

The amplitude ( ) and wavelength ( ) of a general waveform. as the distance between two consecutive corresponding푦 are 휆(i.e., the wavelength) and a stopwatch to measure how points on a wave—for example, the distance between two much time passes between the arrival of one wave crest successive peaks (Figure 2). and the next (the period). Speed is a distance traveled per unit time, and in the case of a wave the distance is the A wave can also be described in terms of its frequency wavelength, and the time is the period. , which is defined as the number of oscillations it makes in a given amount of time. (For instance, think of how many distance 푓 = = Northwest Pa. Collegiate Academy - Erie, PA ripples pass by a certain point in a pond every second.) time The standard unit of frequency is the hertz (Hz), which 휆 Recalling that period푣 is the reciprocal of frequency, we can equals one oscillation per second. The period of a wave 푇 is the amount of time that elapses between oscillations convert this equation to the often more useful form: 푇 and is related to the frequency by . For example, if = . the frequency of a wave is 10 Hz (i.e., 10 oscillations per second), the corresponding period� is= 0.1 1/푓 seconds because This relationship applies to푣 any푓휆 kind of wave, from light to each oscillation is 1/10 seconds apart. sound to water. Notice that according to this equation, the higher the frequency of a wave, the shorter its wavelength. Another important characteristic of a wave is the wave speed . The speed of any wave is related to both its ELECTROMAGNETIC WAVES frequency and its wavelength. Let’s determine the exact Light is an electromagnetic wave, which is a wave 푣 relationship by considering the motion of a wave past a made up of electric and magnetic fields that oscillate as fixed point, such as crests of water past the edge of a dock. the wave travels (Figure 3). Although these electric and We can use a ruler to measure how far apart the crests magnetic fields are continuously changing in magnitude,

2018–2019 Science6 Resource Guide FIGURE 3

An electromagnetic wave is made up of oscillating electric and magnetic fields. The wave moves in the direction perpendicular to both oscillating fields. Northwest Pa. Collegiate Academy - Erie, PA they are always directed perpendicular to one another. charged particle accelerates, the fluctuation in its electric Furthermore, the fields are always directed perpendicular field causes a self-perpetuating “pulse” that spreads out like to the direction of wave propagation. For this reason, the ripples in a pond. In the early 1860s, Scottish physicist electromagnetic waves are classified as transverse. James Clerk Maxwell derived a system of equations that Electromagnetic waves also do not require a physical describes the behavior of electric and magnetic fields, medium through which to propagate. Unlike mechanical including how a changing electric field can generate a waves, such as sound, electromagnetic waves can travel magnetic field and vice versa. In a paper presented to through the empty vacuum of space. This is how light from the Royal Society in 1864, Maxwell used these equations the Sun and other stars reaches Earth. to predict the existence of electromagnetic waves. When a sinusoidally-oscillating charged particle creates an We mentioned earlier that waves are caused by some kind oscillating electric field, then an oscillating perpendicular of initial vibration. Electromagnetic waves are caused by magnetic field is also created. In conjunction, these the vibrations of charged particles, such as electrons and two mutually perpendicular oscillating fields form an protons. Charged particles are surrounded by electric electromagnetic wave. fields that exert forces on other charged particles. When a

2018–2019 Science7 Resource Guide THE SPEED OF LIGHT By shining a beam of light through the teeth of the turning Electromagnetic waves traveling through empty space cogwheel and observing its reflection in the distant mirror, never speed up or slow down. Indeed, it is a fundamental Fizeau’s experiment served as a rapid simulation of the tenet of physics that all electromagnetic waves move at covering and uncovering of Galileo’s lanterns (Figure 5). the same speed through a vacuum. Why should this be Using this apparatus, Fizeau obtained a value for equal so? Suppose an electromagnetic wave were to gradually to 313,000,000 m/s. In 1862, Fellow French physicist Leon lose speed. According to the laws of electromagnetism, the Foucault improved upon Fizeau’s result by replacing푐 the oscillating electric field would generate a weaker magnetic cogwheel with a rotating mirror, yielding a more accurate field, which would in turn generate a weaker electric field, and so on. In short, the wave would die out, and energy FIGURE 4 would be lost—a violation of the principle of energy conservation. Using the same logic, an electromagnetic wave could not speed up because the electric and magnetic fields would continually strengthen one another, and the energy of the wave would increase without end. As it turns out, there is only one speed at which the electric and magnetic fields perfectly self-perpetuate in a manner consistent with energy conservation. This speed, known as the speed of light, , is equal to 299,792,458 m/s.1

Before the 1600s, prominent푐 scientists, including Johannes Kepler and Rene Descartes, believed light to be transmitted instantaneously. Galileo Galilei was among the first to question this viewpoint. To test whether the speed of light could be measured, Galileo proposed an experiment in which two participants would hold covered lanterns a great distance apart. One person would uncover his lantern, and then the second person would uncover his lantern the instant he saw the light from the first lantern. By knowing the distance separating the two lanterns, Galileo could then determine the speed of light. However, limited by both the timekeeping methods of the time and human reaction speeds, Galileo could only estimate that light was at least ten times faster than sound.2

In 1676, Danish astronomer Ole Römer was studying the moons of Jupiter when he made an interesting observation Northwest Pa. Collegiate Academy - Erie, PA regarding their eclipses. Römer noticed that eclipses of Io tended to happen later than expected when Earth was farther from Jupiter, and earlier than expected when Earth was closer to Jupiter (Figure 4). He attributed the difference to the greater distance the light from Io had to travel to reach Earth. Using the then-accepted value for the diameter of Earth’s orbit, Römer calculated the speed of light to be about 220,000,000 m/s, the first quantitative measurement of .3

Around 1850, French physicist Hippolyte Fizeau sought푐 to measure the speed of light via an experiment on Earth— Illustration from Ole Römer’s original 1676 publication akin to Galileo’s lantern experiment centuries earlier. regarding the speed of light. The largest circle is Fizeau constructed an apparatus consisting of a cogwheel Earth’s orbit around the Sun (A). Jupiter is located at and a mirror separated by a distance of eight kilometers. (B), eclipsing its moon Io at points (DC).

2018–2019 Science8 Resource Guide value of 298,000,000 m/s.4 Foucault’s method, and his final measurements in 1931 yielded an even more accurate value of 299,774,000 m/s.6 It’s important to note that even by the time of Foucault’s Today the speed of light is one of the most well-established experiment, light was not yet understood to be an values in all of physics. We know its value to such precision electromagnetic wave. In an 1865 paper, James Maxwell that we now define the length of the meter in terms of it. used his system of equations to determine a theoretical value for the speed of electromagnetic waves. He obtained THE ELECTROMAGNETIC SPECTRUM a value of 300,000,000 m/s, in astonishingly close We are constantly surrounded by an undulating sea of agreement with the speed of light that had been measured electromagnetic waves, with each wave characterized only years prior. Maxwell concluded that light was, in fact, by its own amplitude and frequency. In addition to visible an electromagnetic wave: “The agreement of the results light, the space around us is filled with a multitude of seems to show that light and magnetism are affections of waves with different wavelengths, from radio waves to the same substance, and that light is an electromagnetic gamma rays. This continuum of electromagnetic waves, disturbance propagated through the field according to arranged by either frequency or wavelength, is called the 5 electromagnetic laws.” electromagnetic spectrum (Figure 6).

Subsequent measurements of the speed of light were It’s worth noting that the labels for each category serve conducted by Albert Michelson over a half-century period as a historic classification system and do not indicate beginning in 1879. Michelson’s techniques improved upon fundamental differences between each type of wave. Each

FIGURE 5 Northwest Pa. Collegiate Academy - Erie, PA

Fizeau’s speed of light measurement apparatus.

2018–2019 Science9 Resource Guide FIGURE 6

The electromagnetic spectrum. kind of wave in the spectrum is a manifestation of the same ±± Ultraviolet (UV) light ranges from about 400 nm to electromagnetic wave phenomenon, differing from one 10 nm. The Sun is a significant source of UV light another in their frequency of oscillation, which is directly received here on Earth although most is absorbed related to the amount of energy they are capable of by the atmosphere. UV light in large doses is depositing in matter. There is no sharp boundary between harmful to humans, but it also has practical uses in one category of electromagnetic wave and the next. the sterilization of medical instruments.

–4 ±± Radio waves have the longest wavelengths in ±± X-rays have wavelengths between about 10 nm the electromagnetic spectrum, typically 30 cm and 10 nm. The most familiar application of X-rays or longer. Some AM radio waves even have is in medicine for the examination of bones, teeth, wavelengths as long as 1 km. Radio waves and organs. X-rays are energetic enough to be are primarily used in radio and television harmful to living tissue—you may recall wearing communication, but also in radar systems, satellites, thick shielding to protect your body when having and walkie-talkies. an X-ray performed at the doctor or dentist’s office. ±± Microwaves have wavelengths between 1 mm and ±± Gamma rays are short-wavelength, highly 30 cm. Although you are likely most familiar with penetrating waves that are emitted by radioactive –14 their application to cooking food in microwave nuclei. They have wavelengths between 10 m –10 ovens, microwaves are also used for aircraft and 10 m. Gamma rays are extremely hazardous to living tissue and can cause irreversible damage

navigation, radar, and scientific research. Northwest Pa. Collegiate Academy - Erie, PA if absorbed. However, the destructive capability ±± Infrared (IR) waves have wavelengths longer than of gamma rays can also be harnessed to destroy visible light, between about 700 nm and 1 mm. IR harmful cells such as cancerous tumors. waves are readily absorbed and emitted by most materials. When absorbed, IR waves cause objects ANTENNAS AND RADIO WAVES to get hotter by increasing the kinetic energy of Antennas can be used to transmit and receive atoms and molecules. Infrared has a wide range of electromagnetic waves. When electrons in an antenna applications in meteorology, night-vision devices, are moved back and forth at a given frequency, an communication, and astronomy, to name a few. electromagnetic wave is produced with a matching ±± Visible light is the set of wavelengths detectable by frequency. For example, if the frequency of your favorite the human eye. The colors of the visible spectrum radio station is 94.9 MHz, that means the electromagnetic range from violet (≈400 nm) to red (≈700 nm). waves produced by the station’s antenna (and received Human eyes are not equally sensitive to all by your radio’s antenna) are oscillating at a frequency of wavelengths; the peak of sensitivity is around 560 94,900,000 cycles per second. By tuning your radio to the nm (yellow-green light). same frequency, you can receive a clear signal from that

2018–2019 Science10 Resource Guide FIGURE 7

A radio wave produced by a station antenna is received by a home antenna. You can generate your own radio wave pulses using the “Radio Waves & Electromagnetic Fields” PhET applet at phet.colorado.edu.

Photo Credit: PhET applet at phet.colorado.edu. Northwest Pa. Collegiate Academy - Erie, PA

electromagnetic wave (Figure 7). At the time of his death in 1894, Hertz believed his radio waves served no practical purpose beyond verifying In 1887, German physicist Heinrich Hertz generated the Maxwell’s equations. Within a few years, however, the full first radio waves in a laboratory. In a series of subsequent potential of radio waves was made clear. In the mid-1890s, experiments, Hertz demonstrated that even though the Italian engineer Guglielmo Marconi revolutionized the world waves could not be seen, they exhibited many similar by harnessing radio waves for the purposes of wireless properties to light waves. For instance, Hertz showed that communication. Marconi made the first public demonstration these waves traveled at the speed of light, and he was able of a radio transmitter and receiver in July 1896. to focus and redirect them using reflective surfaces. (Hertz made numerous contributions to experimental physics. You TRANSPARENT vs. OPAQUE MATERIALS may have noticed that the unit of frequency, the hertz, is How is it possible for light waves to pass through a glass named in his honor.) window but not a piece of plywood? To answer this

2018–2019 Science11 Resource Guide question, we must zoom in to examine the atomic building does in a vacuum. blocks that make up the world around us. Everyday materials are composed of molecules, which are in turn In contrast, if the material is opaque, the wave will not made up of atoms containing electrons. Although electrons be passed through the inside of the material. Instead, the are generally bound to a particular atom, the oscillating electrons closest to the surface of the object will vibrate electric fields of an incident electromagnetic wave can briefly in response to the light and then re-emit a reflected cause an atom’s outer electrons to vibrate back and forth. light wave back outward. Most objects around us are opaque, and we only see the outside of them due to the Many objects have a natural frequency at which they light that is re-emitted back from the electrons on the tend to oscillate—think about the specific tone a tuning surface (or, in other words, reflected). fork makes when struck, or the rate at which a playground swing moves back and forth when displaced. When these Metals tend to be opaque to most electromagnetic objects encounter a periodic disturbance that matches their waves. The outermost electrons in metals are not bound natural frequency, their amplitude of oscillation increases to particular atoms and are free to migrate throughout the strongly. This phenomenon is called resonance. You may material, which explains why metals tend to be particularly have encountered resonance when pushing someone in a good conductors of both electricity and heat. When light swing with just the right timing to make the swing go higher, is incident upon the surface of a metal, these free electrons or—more impressively—when observing how a crystal respond more to the electric fields in the wave than they wineglass can shatter when exposed to a musical tone that would if they were bound to a particular atom. As a result, matches its natural frequency. In the same way, electrons the electrons redistribute themselves in such a way that within the molecules of everyday matter have natural they set up an electric field that exactly cancels the field of frequencies of oscillation; the exact frequency is dependent the incident wave. (The ability of electrons to redistribute upon the molecular composition of the material. in this way is a defining characteristic of conductors.) This redistribution produces an outgoing wave that matches the When light of a given frequency is incident upon a material frequency of the incoming wave, which is the reflected light with electrons having the same resonant frequency, the light that we see from a shiny metal surface. If there is resistance wave is absorbed. The energy of the incident light wave is to the motion of the electrons, the result will be less reflected converted into thermal energy of the atom, which collides light and a duller appearance to the metal. and interacts with neighboring atoms, thereby raising the temperature of the material. Materials that absorb VISIBLE LIGHT AND COLOR particular frequencies of light are said to be opaque to those frequencies. Ordinary window glass is opaque to THE VISIBLE SPECTRUM UV light because electrons within glass atoms tend to have The colors of light we see depend on the frequencies of natural frequencies in the ultraviolet range. Accordingly, light that reach our eyes. The lowest-frequency light in the when UV light is incident upon glass, it warms the glass visible spectrum appears to most people as red and the instead of being transmitted.

highest-frequency light as violet. Although there are an Northwest Pa. Collegiate Academy - Erie, PA What happens if incident light does not match the resonant infinite number of frequencies within the visible spectrum, frequency of a material? The electrons are still set into conventionally we often group them into the seven colors vibration, but at lower amplitudes for shorter periods of Red, Orange, Yellow, Green, Blue, Indigo, and Violet. You time. The vibrating electrons then generate their own can use the memory device “ROYGBIV” (often pronounced electromagnetic waves that propagate outward, and like “Roy G. Biv”) to remember the colors of the visible there is less energy converted into heat. If the material is spectrum in order from lowest to highest frequency. transparent, the reemitted waves will be continuously It’s important to note that although we often describe light absorbed and emitted by neighboring atoms until the wave as “blue” or “red” based on its frequency, color is not an exits from the opposite side of the glass. Throughout this inherent property of any electromagnetic wave. Nor is the process, the frequency of the light does not change as it color of an object on or within the object itself. Color is a is “handed off” between atoms. There is, however, a time physiological experience, involving our eyes and brain, delay on the order of nanoseconds between when each that differs from person to person. For instance, even if the atom absorbs and reemits a wave. Consequently, light same light waves reached their eyes, a person with color passes more slowly through transparent materials than it blindness might perceive those waves differently from a

2018–2019 Science12 Resource Guide person with normal color vision. campfires, lamps, and lasers, are sources of visible light. We see them because they emit electromagnetic waves Light that is made up of a mixture of all colors in the visible in the visible spectrum as the result of electrons being spectrum appears white. For example, white light from the rearranged within atoms and molecules. However, we also Sun is a composite of all visible wavelengths of light. In see objects that aren’t sources of light, but only because the late 1660s, Sir Isaac Newton first demonstrated that they reflect light from other sources to our eyes. white light is a composite of colored light by using a prism to separate sunlight into a spectrum of colors and then Objects absorb light of certain frequencies and reflect the recombining them with a second prism (Figure 8). (Later rest. The frequency of the reflected light determines the we will discuss how a prism separates out individual colors apparent color of the object. A red apple, for instance, from white light.) appears red because it absorbs all frequencies of light incident upon it except red, which it reflects. Materials that The Sun emits a range of electromagnetic wave reflect light of all visible frequencies appear to be the same frequencies, the vast majority (over 99 percent) belonging color as the light that is incident upon them. Conversely, to ultraviolet, visible, and infrared portions of the spectrum. objects that absorb all incident light reflect no visible The peak of solar intensity resides squarely within the visible frequencies and appear black. spectrum for humans. That’s certainly no coincidence— evolutionary processes over millions of years have caused It’s important to note that an object can only reflect human eyes to be most sensitive to the frequencies of light frequencies that exist within the light that is incident upon that are emitted most strongly by the Sun. Some animals it. In other words, the apparent color of an object is and insects are sensitive to electromagnetic waves that are dependent upon what kind of light illuminates it. Consider beyond the visible spectrum for humans. Bees, for example, an apple that appears red when illuminated by white light. are sensitive to a portion of the solar spectrum that extends Under red light, the apple will still appear red because red into the ultraviolet, which enables them to detect UV wavelengths are clearly still able to be reflected. However, patterns that direct them toward pollination sites on certain under green light, the apple will appear black. Why? The species of flowers. green wavelengths of light are completely absorbed by the apple, and none are reflected to the observer F( igure SELECTIVE ABSORPTION AND 9). Incandescent bulbs are often described as producing REFLECTION a “warmer” light than fluorescent bulbs because the light What does it mean for an object to appear red or yellow waves they emit tend to be mostly from the low-frequency or violet? Some objects we see, such as the Sun, the stars, (i.e., red) side of the visual spectrum. As a result, parts

FIGURE 8 Northwest Pa. Collegiate Academy - Erie, PA

An illustration of Newton’s double prism experiment, taken from a 1671 letter to the Royal Society.

2018–2019 Science13 Resource Guide FIGURE 9

A red apple reflects red light and absorbs the rest. Under red light, the leaves appear black. Under green light, the apple appears black.

of objects that reflect red light tend to be more visually reflected, but instead is scattered off the molecules that prominent when illuminated by an incandescent bulb. make up the atmosphere. Scattering in this context refers to light being absorbed and re-emitted by the atmospheric SELECTIVE TRANSMISSION molecules. Ordinary window glass appears colorless because it transmits all frequencies of visible light equally well. It’s Earth’s atmosphere is made up of molecular gases— also possible for materials to absorb certain frequencies primarily nitrogen (N2) and oxygen (O2), but also argon of visible light but allow others to be transmitted. Rubies, (Ar), carbon dioxide (CO2), and other gases in trace for example, are a variety of the mineral corundum, which amounts. The N2 and O2 molecules have resonant consists of aluminum oxide in a crystalline structure. In its frequencies at the high-frequency end of the visible purest form, corundum is colorless. The red color of rubies is spectrum. As a result, these molecules tend to scatter blue due to chromium impurities scattered throughout the crystal light from the Sun more strongly than they scatter red light. structure, which absorb frequencies of light within the visible The scattered blue light is sent in all directions, some of spectrum except red. Interestingly, sapphires also belong to the corundum family, but instead contain titanium and iron FIGURE 10 impurities that lend them their stunning blue color. Northwest Pa. Collegiate Academy - Erie, PA It’s also possible to artificially produce materials that are transparent only to certain frequencies. For example, colored glass is produced by adding metals and metal oxides during the manufacturing process. Cobalt blue glass is known for its deep blue coloring, which is due to the cobalt oxide it contains (Figure 10). BLUE SKIES AND RED SUNSETS Using the ideas we’ve covered so far, you have enough information to answer an age-old question: Why is the sky blue? As we discussed earlier, the colors we see are caused by objects selectively reflecting certain frequencies of light to our eyes and absorbing the rest. But how can air reflect light at all? Strictly speaking, the light is not exactly Cobalt blue glassware.

2018–2019 Science14 Resource Guide which reaches our eyes (Figure 11). The result is a vibrant blue sky. This effect is known as Rayleigh scattering after its discoverer, British physicist Lord Rayleigh. Violet Try It! light is actually scattered more strongly by atmospheric particles than blue light, but our eyes are more sensitive to Sunset in a Glass blue than violet. Obtain a clear drinking glass, water, milk, and a bright Of course, the sky does not always appear blue. In your flashlight (LED light works best). Fill the glass with water lifetime, you’ve likely witnessed many breathtaking sunsets, and then stir in about ½ tablespoon of milk. Point the lit made up of deep reds and oranges, as the Sun dips close flashlight directly at the glass from the side, and move to the horizon. At sunset, light from the Sun has to travel your head around the glass to view the scattered light further through the atmosphere in order to reach the from all angles. The light should appear blue when observer. By the time the light from the setting Sun reaches viewed from the sides of the glass at right angles to the your eyes, the higher frequencies of light have scattered flashlight, and red when viewed straight on from the away, whereas the lower frequencies (corresponding to side opposite the flashlight. In this experiment, the light the red end of the visible spectrum) have not (Figure 12). from the flashlight is scattering off the milk particles What’s left are the brilliant, warm colors of the sunset. suspended in the water in a manner similar to how light scatters off particles in the atmosphere.

FIGURE 11 Northwest Pa. Collegiate Academy - Erie, PA

The sky appears blue because blue light is scattered the most toward our eyes due to Rayleigh scattering.

2018–2019 Science15 Resource Guide FIGURE 12

The red colors of sunsets are what remain after the blue frequencies have scattered away.

SECTION I SUMMARY to communication in 1896. ±± Transparent objects allow incoming light waves ±± A wave is the motion of a disturbance between to be transmitted through them. Opaque objects two points. All waves transport energy but not absorb incoming light and reflect it back at their mass. Light is an electromagnetic wave, made surface. up of oscillating electric and magnetic fields. Electromagnetic waves are produced when ±± The visible spectrum represents a small fraction of the entire electromagnetic spectrum. The colors we

charged particles accelerate. Northwest Pa. Collegiate Academy - Erie, PA see depend on the frequencies of light that reach ±± All electromagnetic waves travel at the same our eyes. White light is a composite of all visible speed, the speed of light ( ), in empty space. The wavelengths of light and can be separated into a speed of light is equal to 299,792,458 m/s. full spectrum using a prism. 푐 ±± The electromagnetic spectrum is a classification ±± Objects absorb certain frequencies of light and of all electromagnetic waves by frequency. Each reflect the rest. The frequency of the reflected type of wave in the spectrum represents the same light determines the apparent color of the object. electromagnetic phenomenon, differing from one Objects that absorb all incident frequencies reflect another only in their frequency of oscillation. no light and appear black. ±± An antenna is a means of producing ±± The sky appears blue due to Rayleigh scattering electromagnetic waves of a particular frequency of sunlight from atmospheric molecules. Higher by moving electrons back and forth. Heinrich frequencies within the visible spectrum (e.g., blue) Hertz produced the first radio waves in 1887, and are scattered more strongly toward observers. Guglielmo Marconi demonstrated their application

2018–2019 Science16 Resource Guide Section II

Optics

INTRODUCTION absorb at least some incoming light and reflect the rest. Light can be made more useful when it is shaped, The Law of Reflection redirected, and focused by mirrors and lenses. As we will see, lasers use mirrors and lenses for their basic operation Consider a light ray that is incident upon a smooth, as well as to make them more useful for practical purposes. reflective surface F( igure 13). The incoming ray makes an angle , known as the angle of incidence, with an Optics is a branch of physics involving the properties of i light and how light interacts with matter through reflection, imaginary line perpendicular to the reflecting surface. Ө refraction, and diffraction. In addition to lasers, optics plays This line is called the normal. (In physics, we often use an important role in the function of cameras, magnifiers, the word normal as another word for perpendicular.) The outgoing ray makes an angle , called the angle telescopes, microscopes, and many other devices we use r on a daily basis. of reflection, with the normal. According to the law of reflection, the angle of incidenceӨ is equal to the angle of reflection. Symbolically, this can be written as follows: GEOMETRIC OPTICS

i r , THE RAY APPROXIMATION where is the angle of incidence and is the angle of Centuries of observations and experiments have i Ө = Ө r demonstrated that light travels in a straight line until it reflection. To put this in words, the angle that a light ray Ө Ө encounters a boundary with another medium. This statement makes as it reflects from a surface is always equal to the generally holds true provided that the light is interacting angle the incoming light ray makes with the surface. with objects much larger than its wavelength. At these length scales, we can represent beams of light as rays, or FIGURE 13 straight lines with a direction. This simplification is called the ray approximation. The part of optics in which the ray

approximation is valid is known as geometric optics, or Northwest Pa. Collegiate Academy - Erie, PA ray optics. Throughout most of this section, we will apply the ray approximation when analyzing the path that light waves follow through various materials. Later in this section, we will consider the behavior of waves when the ray approximation is no longer valid. REFLECTION As we described in Section I, most objects we see do not emit light of their own, but rather they re-emit light that is incident upon their surface from a source of light. When light reaches the surface of a material it is either absorbed, transmitted through the material, or reflected.Reflection occurs when light is not absorbed or fully transmitted, and some portion The law of reflection states that the angle of incidence of the wave changes direction at the surface of the material ( i ) equals the angle of reflection ( r ). without actually entering the material itself. All substances Ө Ө

2018–2019 Science17 Resource Guide FIGURE 14

Specular reflection (left) and diffuse reflection (right).

Specular and Diffuse Reflection cause visible light to be diffusely reflected, but they also act as specular reflectors for long-wavelength radio waves. Reflection of light from a smooth surface is calledspecular reflection. During specular reflection, incident light rays REFRACTION remain parallel to one another, resulting in a mirror-like When a light ray encounters a boundary between two reflection of incoming light. A mirror is, of course, an excellent different transparent materials, the transmitted ray changes example of a specular reflector, but specular reflection can direction in a process called refraction (Figure 15). also be observed from window glass or still water. Refraction occurs because light travels at different speeds on each side of the boundary. The refracted path taken by Surfaces do not need to be smooth in order to cause light entering a different medium can be described using reflection. Reflection from a rough surface is calleddiffuse Fermat’s principle of least time. In 1662, French . As you can see in Figure 14, the law of reflection reflection mathematician Pierre de Fermat proposed that the path that still applies to each incoming light ray; however, the light takes between two points is the path that minimizes the irregularity of the surface causes each individual ray to be

travel time. For two points within the same material, this is Northwest Pa. Collegiate Academy - Erie, PA reflected in a slightly different direction. The reflection of light obviously the straight-line path connecting them. But what from most walls is an example of diffuse reflection; walls tend about two points in two different materials? to reflect light back into a room, but they are (generally) not shiny and do not form images as mirrors do. Most objects We can use an analogy to think about how Fermat’s around us are visible as a result of diffuse reflection. principle applies to refraction. Imagine you are serving as a lifeguard on a beach when you suddenly notice a swimmer The “smoothness” of a surface is relative to the wavelength in distress (Figure 16). Your running speed on the beach is of the wave that is being reflected. If the height of the faster than your swimming speed in the water. Which path irregularities on a surface are small compared to the will allow you to reach the swimmer in the least amount of wavelength of the incident light, the surface will appear time? Path A requires a greater distance traveled through smooth to the wave and it will undergo specular reflection. water than on the beach, so it can’t be the fastest route. Therefore, it’s possible for the same surface to appear rough Although path B is a straight line, you could reduce the to one wave and to appear smooth to another wave with a overall travel time by covering more distance over land and longer wavelength. For example, the dish surfaces of large less through water. Without going into a formal derivation, radio telescopes, some more than fifty meters in diameter, path C is the choice that minimizes the time to reach the

2018–2019 Science18 Resource Guide FIGURE 15 TABLE 1

Medium Index of Refraction Vacuum 1 (exact) Air 1.0003 Water 1.33 Crown Glass 1.52 Sapphire 1.77 Diamond 2.42

A light ray refracting as it enters a plastic block, and Indices of refraction for various media. again as it leaves. The Law of Refraction

FIGURE 16 We stated previously that in a vacuum devoid of any matter, light always travels at the speed of light, c. When light passes through a transparent medium such as air, water, or glass, it slows down by a factor characteristic to that medium called the index of refraction. The index of refraction, n, of a material is defined as the speed of light in a vacuum,c , divided by the speed of light in the material, v: c n v speed of light in a vacuum = = Rearranging, wespeed can of see light that in the the speed material of light within a material with index of refraction n is given by v = c/n. A few points are worth noting:

±± n is a dimensionless number (i.e., it has no units). Northwest Pa. Collegiate Academy - Erie, PA ±± n is always greater than or equal to 1. Three paths a lifeguard could take to reach a swimmer ±± n is defined to be 1 for a vacuum. in distress. The least-time path, C, resembles the path of Table 1 shows the index of refraction for some common a light ray entering a medium in which it moves more materials. In general, light travels more slowly in materials slowly. with higher indices of refraction. Correspondingly, a light ray will bend more when passing from a vacuum into a material swimmer. Likewise, a light ray entering a material in which it with a higher index of refraction than it would when passing moves more slowly will follow a bent path similar to path C. into a material with a lower index of refraction. Just as the optimal path for a lifeguard would depend on his or her swimming and running speeds, the exact angle the When a light wave passes between two materials, its ray follows will depend on the speed at which light travels frequency does not change. Without going into a great in each material. deal of detail, this is because a light wave must be “continuous” as it passes between regions of differing

2018–2019 Science19 Resource Guide lower index of refraction, it bends away from the normal. FIGURE 17 Mirages Have you ever been traveling in a car on a hot day and noticed what appear to be pools of water covering the road in the distance? As you continue to drive, you never reach the water despite the very convincing image you saw only moments before. This is a familiar example of a mirage, which is a naturally occurring optical illusion due to the refraction of light through the atmosphere. The air just above the surface of the roadway is very hot compared to air higher up, and light travels faster through the hot air because it is less dense. As a result, light that is incident upon the road ahead of you is refracted back upward toward your eyes (Figure 18). The fact that the incoming light is behaving in a manner consistent with reflection— although the effect is entirely due to refraction—causes your brain to interpret the region above the road as a reflective surface such as water.

We often associate mirages with the desert, as the hot The law of refraction. temperatures cause varying air densities that refract light © OpenStax CNX / OpenStax.org / CC-BY-SA-4.0 through various angles. This causes a wavy, shimmering effect that can also be seen above a hot grill or stove. refractive indices. That is, in a given period of time, Although it is common to think of mirages as being the number of wave crests passing into one side of an hallucinations or “tricks of the mind,” they are the result of interface must equal the number of wave crests leaving true refractive behavior of light rays and can therefore be the other side. Recall from Section I that for a wave, the photographed. speed, frequency, and wavelength are related by v = f . Accordingly, when the speed of a light wave is reduced A similar refractive effect causes the Sun to be visible at within a transparent medium, its wavelength must also be휆 the horizon even after it has “set.” Earth’s atmosphere is reduced by the same factor. If a wave has a wavelength in a vacuum, its wavelength in a medium with index of FIGURE 18 refraction n will be /n. 휆

The equation relating휆 the angle of incidence and the angle Northwest Pa. Collegiate Academy - Erie, PA of refraction is known as Snell’s law after seventeenth- century Dutch astronomer and mathematician W. Snell. According to Snell’s law, when a light ray travels from a region with index of refraction n1 to a region with an index of refraction n2 , the angle of incidence 1 is related to the refracted angle 2 by the following equation: Ө Ө n1sin 1 = n2sin 2.

Although we won’t provideӨ a full mathematicalӨ treatment of Snell’s law, we will describe some general rules of refraction. When a light ray enters a medium with a higher index of refraction, it bends toward the normal. Furthermore, the A mirage of distant pools of water on a hot roadway, greater the refractive index, the more the light ray will bend caused by refraction. toward the normal. When a light ray enters a medium with a

2018–2019 Science20 Resource Guide less dense at higher altitudes; accordingly, the least-time of refraction as a result. Dispersion is the separation of pathway for sunlight is a curved arc that favors higher visible light into a spectrum of colors due to this dependence altitudes where it travels more quickly. The curved path of refractive index on wavelength. Dispersion is commonly is due to the fact that the index of refraction changes illustrated using a prism, which is a piece of glass or gradually, layer by layer, in the atmosphere. To an observer plastic typically cut into a triangular block. When white on Earth’s surface, the image of the Sun persists at the light is passed through one of the angled faces of a prism, a horizon even after the Sun itself should technically be out spectrum of colors is produced through dispersion. of view (Figure 19). The image of the Sun may also appear “squished” vertically just as it disappears from sight due to Rainbows the same effect of refraction. Rainbows are caused by millions of raindrops acting as tiny DISPERSION prisms dispersing sunlight. A light ray from the Sun enters the Section I described how light behaves within transparent droplet and is refracted. At the far side of the droplet, the materials by introducing the principle of resonance. Light is ray is reflected and returns to the front side, where it once mediated by electron oscillators within materials. Electron again is refracted as it passes back into the surrounding air oscillators in most transparent materials, such as glass, (Figure 20a). The slight difference in angle between each resonate most strongly with frequencies of light in the wavelength of light causes the vivid spectrum of colors that ultraviolet range. Therefore, light waves with frequencies we see. Although each droplet produces a full spectrum closer to the violet end of the visible spectrum are more likely of colors, an observer’s eye is only positioned to see one to interact with oscillators in these materials and travel more of those colors. The droplets at the very top of the rainbow slowly on average as a result. Indeed, violet light travels direct red light toward the observer; violet light dispersed about 1 percent more slowly in ordinary glass than red light. from this droplet passes above the observer’s eye. Likewise, The index of refraction is slightly greater for violet light than the droplets at the bottom edge of the rainbow direct violet red light, and violet light will undergo a greater amount light toward the observer (Figure 20b).

FIGURE 19 Northwest Pa. Collegiate Academy - Erie, PA

The Sun appears to linger at the horizon due to refraction through Earth’s atmosphere.

2018–2019 Science21 Resource Guide In order for a rainbow to be visible, water droplets (in a TOTAL INTERNAL REFLECTION cloud or falling rain) must be present in one part of the In general, when light moves between two different sky, and the Sun must be shining in the opposite part of transparent materials, some light is refracted and some the sky. (This is the reason rainbows are often associated is reflected.Total internal reflection is an optical with the passing of storms.) On a clear day, you can create phenomenon in which a light ray traveling from a region of your own rainbow by viewing a cascade of droplets from higher refractive index to a region of lower refractive index a garden hose or lawn sprinkler. An observer positioned is completely reflected at the boundary. Consider what between the Sun and the water droplets, facing away from happens as the angle of incidence is gradually increased the Sun, will see a spectrum of colors in a circular arc. If (Figure 21): the angle of refraction also gradually increases it weren’t for the ground getting in the way, you would until, at the critical angle, the refracted ray will be oriented actually see the rainbow trace out a full circle—in fact, at 90° relative to the normal. For all angles of incidence under the right conditions, it’s possible to view a circular greater than the critical angle, no refracted ray will be rainbow from an airplane or very tall building.7 produced, and the ray will be entirely reflected back into the medium. The reflected light ray obeys the law of reflection. A secondary, or “double,” rainbow is a rare sight requiring just the right conditions to view. The secondary The effect of total internal reflection explains why diamonds rainbow appears above the primary and results when sparkle more than other materials. Because diamond has a light undergoes an additional reflection before exiting the relatively high index of refraction (typically 2.42), the critical droplet. This second reflection causes the order of colors angle is small (about 24.4°), and therefore light rays typically in the secondary rainbow to be reversed. The secondary undergo many internal reflections before striking at a steep rainbow is also much dimmer than the primary due to light enough angle to leave the diamond. Diamonds are typically intensity lost from the additional reflection. The next time shaped by jewelers in such a way that the light is concentrated you see a rainbow, take a moment to appreciate how at only a few places along the surface of the diamond that it refraction, reflection, and dispersion each contribute to this can exit, thereby contributing to the sparkle. Dispersion of light splendid visual phenomenon. also contributes to the brilliant array of colors that are often visible from the surfaces of well-shaped diamonds.

FIGURE 20 Northwest Pa. Collegiate Academy - Erie, PA

(a) Dispersion of light in a spherical water droplet, which causes a rainbow to be visible. (b) The position of colors in the rainbow is caused by colors emerging in different directions.

2018–2019 Science22 Resource Guide FIGURE 21

The critical angle is one for which the angle of refraction is 90°. Above this angle of incidence, no light is refracted, and all light is reflected back into the material.

the length of the fiber even as it bends and twists. Fiber- FIGURE 22 optic cables are flexible, meaning they can transmit light to—and collect images from—otherwise difficult-to-access areas such as a patient’s stomach or the inside of a motor. Optical fibers can even be manufactured smaller than the diameter of a blood vessel.

Fiber-optic cables have several advantages over copper wires and cables in communication. Optical fibers are light, flexible, and can be manufactured at a lower cost compared to conventional communication wiring. Optical fibers also have a greater bandwidth than copper wires, meaning more information can be transmitted per unit time. Unlike electrical signals, light is also not affected by fluctuations in temperature or nearby magnetic fields, so interference and Northwest Pa. Collegiate Academy - Erie, PA “noise” are less of a concern. Finally, optical fibers are more secure than conventional wiring because they are more difficult to tap into by possible eavesdroppers.

MIRRORS AND LENSES Fiber-optic cables are an important application of PLANE MIRRORS total internal reflection. A plane mirror, which has a flat surface, is the simplest type of mirror. Consider a point P on the lid of a pill bottle Fiber-Optics placed in front of a plane mirror (Figure 23). Light rays propagate outward in all directions from point P, and many Another important application of total internal reflection is of these rays reach the mirror. When the rays encounter the optical fiber technology F( igure 22). Optical fibers act as surface of the mirror, they are reflected back according to “pipes” through which light rays undergo a series of total the law of reflection. If we look at the rays diverging from internal reflections. In this way, the light is transmitted along the mirror’s surface, they appear to be emerging from a

2018–2019 Science23 Resource Guide single point (P’), which is the point at which an observer system of visual perception, consisting of our eyes and brain sees an image of the pill bottle lid. We can apply the working together to interpret visual stimuli, cannot ordinarily same reasoning to point Q at the base of the bottle. A little distinguish between an object and the image of that object geometry (and our own experience) shows that the image reflected in a mirror. The illusion of an object existing behind formed by a plane mirror appears as far behind the mirror a mirror is due to the fact that light from a reflected object as the object is in front of the mirror. Furthermore, the size enters the eye in exactly the same manner as if it had been of the image is equal to that of the object. In other words, emitted by a physical object at the location of the image. plane mirrors do not magnify, or enlarge, an image relative to the original object. Ray tracing is the technique of determining or following (tracing) the paths that light rays take in order to determine Images can be classified as real or virtual. A real image the position and size of an image. A diagram (like the one is formed by light rays passing through the location of the in Figure 23) showing the paths that these rays take is called image. If a surface such as a screen is placed at the location a ray diagram. The laws of reflection and refraction, of a real image, the image will be visible. For example, along with some simple approximations, allow us to predict images projected onto a movie theater screen are real the paths the light rays will take. Ray diagrams for curved images, as are images focused by a camera onto film or mirrors and lenses include a principal axis, which is an a photosensitive surface. In contrast, light rays appear to imaginary line passing at a right angle to the surface of diverge from a virtual image, but they do not actually the mirror or lens, through its center. As we will see, ray pass through where the image is formed. For example, the diagrams can help us understand how mirrors and lenses image of the pill bottle in Figure 23 is a virtual image. Our form images in a wide variety of situations.

FIGURE 23 Northwest Pa. Collegiate Academy - Erie, PA

Reflection of an object from a plane mirror. The image is the same size as the object and appears to be the same distance from the mirror as the object.

2018–2019 Science24 Resource Guide FIGURE 24

A ray diagram for an image formed by a concave mirror when the object is outside the focal point (F). Ray 1 enters parallel to the principal axis and reflects through the focal point; ray 2 reflects at equal angles at the center of the mirror; and ray 3 enters through point F and leaves parallel to the principal axis.

© OpenStax CNX / OpenStax.org / CC-BY-SA-4.0 Northwest Pa. Collegiate Academy - Erie, PA CONCAVE AND CONVEX MIRRORS caveat that the normal to the surface will be different at Not all mirrors we use are flat. Cosmetic mirrors, security every point along the surface. mirrors in stores, and passenger side mirrors in cars all have curved surfaces that cause images to be formed in different Concave Mirrors ways compared to plane mirrors. For curved mirrors, the A concave mirror is shaped such that its reflective surface sizes and distances of objects and images are no longer curves inward, away from incident light. Concave mirrors equal, which can cause images to appear warped or reflect incoming parallel light rays inward toward a single distorted. If you’ve ever looked into the reflective surface point, called the focal point, which lies along the principal of a spherical holiday ornament or a curved “funhouse” axis of the mirror. The distance from the surface of the mirror, you may recall seeing a distorted version of your mirror to the focal point is called the focal length, often face or body. We can think of a curved mirror as many abbreviated f. Concave mirrors produce different types of small plane mirrors connected together, each oriented at images depending on the position of the object relative to the a slightly different angle from the one adjacent to it. We mirror. For objects between the mirror and the focal point, can still apply the law of reflection to this situation, with the

2018–2019 Science25 Resource Guide the image is virtual, upright, and magnified. When the object when shaving or applying makeup. Solar cookers also use is located outside the focal point, the resulting image is real concave mirror to focus sunlight at a specific point to cook and inverted (Figure 24). As the object moves farther and food. Concave mirrors are also useful for projecting rays farther away, the light rays arrive at the mirror very nearly straight outward from a light source, as in flashlights and parallel to one another, and the resulting image is formed automobile headlights. As we will discuss in a later section, very close to the focal point of the mirror. We won’t show ray concave mirrors are also used in the construction of optical diagrams for each of these cases here, but you can find them cavities, which are important in laser operation. in most introductory physics texts. Convex Mirrors Since concave mirrors can form upright, magnified images of objects that are within their focal point, they are particularly A convex mirror is shaped such that its reflective surface useful for viewing an enlarged reflection of one’s face curves outward toward the light source. Convex mirrors

FIGURE 25 Northwest Pa. Collegiate Academy - Erie, PA

A ray diagram for an object reflecting from a convex mirror. The rays follow the same rules as in Figure 24, but must be extended through the surface of the mirror. The image is virtual, upright, and smaller than the object.

2018–2019 Science26 Resource Guide also have a focal point, but it is located on the opposite such as hospitals, stores, or schools to allow people to see side of the mirror from the object. Although convex mirrors around the corners of hallways or aisles. Likewise, they reflect light outward and cannot be used to focus light, they are often seen mounted along roadways where there is can create images. Images produced by convex mirrors reduced visibility or the potential for “blind spots” around are always virtual, upright, and reduced (i.e., smaller in turns. Convex mirrors made from blown glass were likely size than the object), as seen in Figure 25. As the object used as early as the fifteenth century, as they are depicted approaches the mirror, the image becomes larger until it is in paintings from that period. Convex mirrors were likely approximately the same size as the object (when the object more common than plane mirrors during that time because is in contact with the mirror). glass-blowing technology of the time made curved mirrors easier to produce.8 You may be familiar with the safety warning “OBJECTS IN MIRROR ARE CLOSER THAN THEY APPEAR” that is LENSES required by law to be etched on passenger side mirrors We have seen how materials can cause rays of light to of motor vehicles in the United States. Convex mirrors bend through refraction. A lens is a carefully shaped are preferable for side mirrors are because they always piece of transparent material, such as plastic or glass, that produce an upright image while providing the driver a refracts light rays to produce an image. Lenses are essential wider field of view than a plane mirror would. However, the to many optical instruments that we use on a regular basis, reduced size of the image may lead the driver to believe including eyeglasses and cameras. We can think of a that the corresponding objects are farther away than they lens as a series of connected prisms, each with a slightly actually are, hence the warning message. different surface angle, that work together to form a bright, Convex mirrors are also often mounted in large buildings focused image of an object. Like curved mirrors, lenses also

FIGURE 26 Northwest Pa. Collegiate Academy - Erie, PA

A ray diagram for an image formed by a converging lens. The three labeled rays correspond to the three principal rays defined in the text.

2018–2019 Science27 Resource Guide have focal points at which they concentrate light. However, enlarged virtual image is formed on the same side of the lenses focus light rays through refraction, not reflection. lens as the object. This case describes a simple magnifier, Lenses also have two focal points, one on each side, which we will discuss later in this section. because light can enter from either face of the lens. Converging Lenses A converging lens, also known as a convex lens, refracts incoming parallel light rays so that they meet, or converge, Try It! at a focal point. Converging lenses can be identified by their shape, as they have convex outer surfaces. In other Make a Pinhole words, they are thicker toward their center than at their edges. An object infinitely far away from a converging lens Camera will create a point image at the focal point. The image is real, which means that it can be projected on a screen. This Instead of lenses, early cameras used small apertures provides a simple method for determining the focal length the size of pinholes to produce an image. You can of a converging lens such as a magnifying glass—use the make your own pinhole camera by removing one side lens to create an image of a distant light source, such as the of a cardboard box and covering the open side with a Sun or a high ceiling light, and measure the distance from semitransparent sheet of tracing or tissue paper. Using a the lens to the image. very thin, sharp instrument such as a pin or needle, poke a hole in the side of the box opposite the paper, and When constructing ray diagrams for lenses, there are three point the hole toward an illuminated object. An inverted light rays that are particularly useful because their behavior image of the object will appear on the sheet of paper is easy to predict using the refracting properties of lenses. opposite the pinhole. The smaller you make the pinhole, These rays are known as principal rays: the sharper the image will be. 1. A light ray traveling parallel to the principal axis (the parallel ray) will be focused at the focal point. This You can even make an actual “camera” by using comes from the definition of focal point. unexposed photographic film in place of the tracing paper, closing the back of the box so that no extra light 2. A ray passing through the center of the lens (the central can enter, and adding a removable flap to cover the ray) will continue in a straight line with no net refraction. pinhole. When you’re ready to take a picture, open the This occurs because both sides of a lens are parallel to flap to allow light from the object to expose the film, one another along any path through the center of the producing a photograph. You’ll need to test various lens. By approximating the lens as having a negligible exposure times depending on the type of film and the thickness, we can assume that the ray does not deviate amount of available light. from a straight line. Northwest Pa. Collegiate Academy - Erie, PA Pinhole cameras are excellent tools for viewing solar 3. A light ray passing through the focal point (the focal eclipses. When the camera is pointed at the Sun, a crisp ray) will leave the lens parallel to the principal axis on image of the eclipse (that’s safe to view) will appear on the opposite side of the lens. the screen. Even without constructing a camera, you can observe a similar “pinhole” effect the next time you are Figure 26 provides an example of how these three rays can be used to predict the location and size of an image for a walking underneath a leafy tree on a sunny day. When converging lens. the spaces between the leaves are small compared to the height of the tree, they act as “pinholes” that project A converging lens can produce a real or a virtual image circular spots of light on the ground. The many circles of an object, depending on where the object is located are each individual images of the Sun cast by different relative to the focal point. As a distant object approaches openings between leaves. the focal point of a converging lens, the image becomes larger and farther away. When the object is between a converging lens and its focal point, the light rays from the object diverge when they pass through the lens, and an

2018–2019 Science28 Resource Guide FIGURE 27

A ray diagram for an image formed by a diverging lens. The three principal rays are labeled.

Diverging Lenses through ray tracing, many more rays pass through the lens and are also refracted to pass through the same image A diverging lens, or concave lens, causes incoming point as the others. Light rays also leave every point on light rays to spread out, or diverge. The focal point of the object (either through emission or reflection) and pass a diverging lens is defined as the point from which the through every part of the lens. What would you expect to diverged rays appear to originate. Diverging lenses can happen to the image of the person in Figure 26 if the top only produce virtual images. When a diverging lens is used half of the lens were covered up? It’s natural to think that alone, the image is always virtual, upright, and reduced an image of half the person would be formed, but this is in size compared to the object. Diverging lenses have incorrect—you would still see a complete image of the concave surfaces; they are thicker toward their edges than person, although it would be fainter because less light is at their center. being focused to produce it. Ray diagrams for diverging lenses use the same principal Spherical and Chromatic Aberration rays as converging lenses. However, these rays may need Northwest Pa. Collegiate Academy - Erie, PA to be extended backward in order to determine the point No lens or mirror produces a perfect image. Due to from which they appear to have originated. Figure 27 refractive and reflective effects, real mirrors and lenses provides an example of a ray diagram for a diverging lens. often behave differently than we might expect according The first ray, parallel to the axis, appears to come from to a simple ray analysis. A distortion of an image caused the focal point on the same side of the lens as the object. by unintended lens or mirror behavior is called an The second ray passes through the center of the lens and aberration. There are two primary types of aberrations: is not refracted. The third ray is drawn as if it were going spherical and chromatic. to the focal point on the opposite site of the lens. As this ray passes through the lens, it is refracted parallel to the Spherical aberration occurs when light passing through the principal axis and must be extended backward, as shown edges of a lens or reflecting from the edges of a mirror is by the dashed line. The location of the tip of the image is focused at a slightly different point than light that is incident the point at which the three rays appear to have originated. near the center (Figure 28), resulting in a blurred image. In a camera, spherical aberration can be reduced by using an It’s important to note that although we have been using adjustable aperture to control the amount of light passing three rays in order to locate the position of an image through each part of the lens. As the aperture size is made

2018–2019 Science29 Resource Guide pupils are at their most narrow, light passes mostly through FIGURE 28 the center of the lens, where spherical and chromatic aberrations are not as pronounced. This is one reason why our vision tends to be much sharper in bright light.

WAVE OPTICS HUYGENS’ PRINCIPLE In the mid-seventeenth century, long before Maxwell proposed the existence of the electromagnetic wave, scientists disagreed about the exact nature of light. Sir Isaac Newton, based on numerous experiments involving prisms and mirrors, argued that light must be made up of weightless particles, called corpuscles, that travel in straight lines and rebound elastically from surfaces. Newton’s theory could account for the basic geometric phenomena of reflection and refraction, but other optical effects could not be adequately explained. For example, light passing through a narrow slit had been observed to spread outward instead of casting a sharp image of the slit, which could not be easily explained with a corpuscle model.

In 1690, Christiaan Huygens, a Dutch physicist and contemporary of Newton, published an alternative wave theory of light. Huygens envisioned light as a Spherical and chromatic aberration. wave that propagates radially outward from a source, © DrBob / Wikimedia Commons / CC-BY-SA-3.0 just as ripples spread outward from a pebble dropped in water. Huygens’ principle states that every point of a wavefront can be treated as itself a source of smaller, only the center of the lens is used to focus light. secondary wavelets that spread out in all directions. However, this also reduces the amount of light being used Although it took several centuries before it became widely to produce an image, so a longer exposure time may be accepted, Huygens’ wave theory is now fundamental necessary to compensate. Spherical aberration in mirrors to our understanding of light. The effects of diffraction, can be corrected by using a parabolic rather than spherical interference, and polarization are all direct consequences mirror. of the wave nature of light. Northwest Pa. Collegiate Academy - Erie, PA Chromatic aberration is caused by light of different colors DIFFRACTION (i.e., wavelengths) being refracted by different amounts by We have already described two ways in which light a lens and, therefore, converging at slightly different points can change its direction of propagation: reflection and (Figure 28). Recall that violet light undergoes a greater refraction. We will cover one more, but it can only be degree of refraction than red light, so violet light accordingly accounted for by considering the wave behavior of light. has a shorter focal length than red light for the same lens. You may have noticed how you can hear sounds even Chromatic aberration can be corrected by combining lenses when the source is around a corner or on the opposite of different types of glass. Although curved mirrors can side of a barrier. This is due to diffraction, which is the cause spherical aberration, they do not introduce chromatic bending of a wave around an edge of a surface or through aberration because no refraction takes place. an aperture (Figure 29). Huygens’ principle helps explain These defects also affect image formation by the lenses of how light spreads through openings by considering every your eyes. Your pupils change size, or dilate, in order to point along the front of the wave to be itself a point source regulate the amount of light entering your eyes. When your of light.

2018–2019 Science30 Resource Guide FIGURE 29

Diffraction of waves through an aperture.

When the size of an opening is large compared with the conventional microscopes can be for viewing very small wavelength of light, it casts a shadow with sharp, well- objects. For instance, most viruses are about 20–300 nm defined edges that resembles the shape of the opening. As in diameter, which is smaller than the wavelengths of visible we narrow the opening to be closer to the wavelength of light. When viewed using a conventional microscope,

light, the edges of the shadow become fuzzier and the light the edges of the virus become indistinct due to diffraction Northwest Pa. Collegiate Academy - Erie, PA emerging from the opening spreads out. Diffraction occurs effects. Electron microscopes, which rely on the fact that for all waves, not just for light or sound. For each kind of all matter exhibits wave-like properties, use fast-moving wave, the same general rule applies: diffraction is most electrons to probe objects at this scale. The wavelength of apparent when the wavelength of the wave is comparable electrons in such a microscope is thousands of times smaller to the opening or obstruction in the path of the wave. than the wavelength of visible light, so diffraction effects from the object under observation are minimized. A good example comes from radio waves. The radio waves used in AM radio have wavelengths approximately INTERFERENCE 100 times longer than those used in FM radio (about 300 meters compared with about 3 meters). Accordingly, Superposition AM radio waves have an easier time diffracting around What happens when two waves in the same medium buildings; AM radio reception is often better than FM radio interact? Traveling waves can meet and even pass through reception in densely populated areas as a result. one another without either individual wave being altered or destroyed. When two waves are in the same place Diffraction effects also place a limitation on how useful

2018–2019 Science31 Resource Guide FIGURE 30

Totally constructive (left) and totally destructive (right) interference of two waves. at the same time, we can add their amplitudes together Double Slit Interference at each individual point to find the amplitude of the resulting wave (Figure 30). This is known as the principle In 1801, Thomas Young investigated a phenomenon known of superposition. In general, the phenomenon of two as double slit interference, in which light passing waves interacting to form a resultant wave is known as through two thin parallel slits produces a pattern of bright interference. Interference effects are possible with any and dark fringes on a screen (Figure 31). This effect is due kind of wave. However, these effects are easiest to observe to alternating constructive and destructive interference, and understand when we consider interference between which can only be explained by treating light as a wave. waves of the same frequency. Light waves exiting each slit have the same phase (i.e., they have crests lined up with crests) but take paths of different When two waves overlap such that their crests and lengths to reach the same point on the screen. Bright fringes troughs align perfectly, we say that they are “in phase,” occur where a crest from one slit reaches the screen at the and the resultant wave has a larger amplitude than either same time as a crest from the other slit. Dark fringes occur of the individual waves. This is called constructive where a crest from one slit reaches the screen at the same interference because the waves build constructively on time as a trough from the other slit, thereby canceling each one another. What happens if we shift one of the waves other out. Young’s discovery provided convincing evidence Northwest Pa. Collegiate Academy - Erie, PA so that the crests of one wave align with the troughs of in favor of Huygens’ wave model of light. another? In this case, adding the amplitudes yields zero at every point along the wave. In other words, the waves We can use a similar analysis to consider diffraction effects cancel each other out in what is known as destructive from a single slit with appreciable width. Recall from interference. Destructive interference between reflected Huygens’ principle that we can treat every point along a sound waves can result in acoustical “dead spots” in wavefront as a point source of light. By applying Huygens’ theaters, where no sound is audible. For this reason, principle to the slit, we can consider individual points along acoustical engineers must design performance spaces to the slit as “in-phase” sources of light. The light “emitted” minimize the effects of destructive interference. Destructive by each tiny part of the slit travels a different distance to interference can be put to practical use in noise-cancelling reach a corresponding point on the viewing screen, and headphones, which measure ambient sound and then therefore they can arrive in or out of phase. Constructive generate an out-of-phase sound wave with the same and destructive interference between these individual amplitude to cancel out the ambient sound. wavelets results in a pattern of bright and dark regions

2018–2019 Science32 Resource Guide FIGURE 31 Northwest Pa. Collegiate Academy - Erie, PA

Top: A simulation of double slit interference. You can explore diffraction and interference effects using the “Wave Interference” PhET applet at phet.colorado.edu. Bottom: The bright and dark fringes are a result of constructive and destructive interference, respectively.

2018–2019 Science33 Resource Guide FIGURE 32 FIGURE 33

A layer of very evenly spaced bacteria floating on Newton’s rings, as observed through a microscope. the surface of a fish tank acts as a natural diffraction grating. or bacterial layers can act as diffraction gratings, which is often useful to scientists in determining their dimensions and along the screen. Effectively, the presence of the slit causes structure (Figure 32). the emerging wave to interfere with itself, producing a diffraction pattern. Thin-Film Interference Diffraction Gratings Long before Young’s double slit experiment, Sir Isaac Newton made observations of interference effects, but Interference patterns can also be created using more than he did not fully grasp their significance. Newton placed a two slits. A diffraction grating uses a large number spherical lens on top of a flat piece of glass, leaving a small (often thousands per centimeter) of equally spaced, parallel air gap between the edges of the two pieces of glass. When slits to produce interference effects. Like prisms, diffraction this combination is illuminated by light, a series of concentric gratings spread light into a spectrum of colors; however, the circular fringes appear when viewed from above. These effect is due to interference and not dispersion. Compared fringes, known as Newton’s rings, are similar to the bright to double slit interference patterns, the interference patterns and dark fringes observed in double slit interference (Figure

produced by diffraction gratings tend to be more spread out, 33). Newton’s rings are caused by incident light moving Northwest Pa. Collegiate Academy - Erie, PA and the dark fringes tend to be broader because the greater through different thicknesses of glass at every point along number of slits provides more opportunities for destructive the lens, and then interfering with light that is reflected from interference to occur. Diffraction gratings can be made by the top of the lens when it re-emerges at the top surface. etching very fine lines on glass using a diamond point; the Newton’s rings are useful in testing the curvature of precisely narrow spaces between the scratches then act as slits. ground lenses; asymmetric ring patterns indicate that the lens is not perfectly spherical and must be reshaped. You’ve probably noticed a spectrum of colors when viewing the underside of a CD or DVD. A CD contains Interference effects are also responsible for the iridescent evenly spaced grooves separated by about 1 micrometer colors seen in a soap bubble or an oil slick. These colors (one millionth of a meter). These grooves act as a diffraction appear when the thickness of the transparent layer is close grating that separates different wavelengths of light toward to the wavelength of light used to illuminate it. Consider your eye. Because the surface is reflective, the resulting multiple rays of light incident on a thin transparent material interference pattern can be viewed simply by looking at the such as a layer of oil floating on water F( igure 34). Some CD. Diffraction effects are also observed in nature; small light waves are reflected from the top surface of the film, repeating formations such as striated muscle tissue, crystals, whereas others are transmitted through the film and reflect

2018–2019 Science34 Resource Guide the transmitted wave travels a different distance through FIGURE 34 the film. The colors on the surface of a soap bubble also change as the bubble becomes thinner and thinner due to evaporation. Thicker walls tend to cancel out longer wavelength light (i.e., red), leaving cyan. As the wall gets thinner, shorter wavelengths such as blue undergo destructive interference. Eventually the bubble is too thin to cause interference of any visible wavelengths of light, and it appears colorless just before popping.

Try It! Thin-Film Interference

The next time you are washing dishes in the sink, you can try making observations of interference effects with thin soap films. Try inverting a cup or mug in soapy water to create a thin film across the opening. What Thin-film interference effects seen in a spill of diesel colors do you see? If you turn the cup sideways, the fuel. The colors magenta, yellow, and cyan are film will flow downward to become thicker toward the prominent. bottom in a narrow “wedge” shape. You should see layers of color corresponding to the varying thickness © John / Wikimedia Commons / CC-BY-SA-2.5 of the film. At the thinnest region near the top of the film, the film appears colorless because no interference in back up from the bottom layer. Because each of these the visible range is occurring. waves travels a different distance, they interfere with one another when they emerge from the top surface of the film. When a thin film is illuminated by white light, the thickness POLARIZATION of the film determines which wavelength of light will Polarization is a property of light that plays an important undergo destructive interference. The light emerging from role in how it interacts with materials. Later we will see that the film at that point will appear as thecomplementary polarization is important to the operation of devices such as color of the light that was canceled. (A complementary lasers. The polarization of an electromagnetic wave refers color is the color that remains when another color is to the plane in which the electric field oscillates as the wave Northwest Pa. Collegiate Academy - Erie, PA subtracted from white light.) For example, if red light is propagates. To help visualize the polarization of a wave, canceled out due to destructive interference, the light that imagine you take a rope attached to a wall at arm height emerges will appear as the complementary color of red, and pull it taut. Then, imagine shaking your arm up and which is cyan. Magenta (the complementary color of down rhythmically to produce a transverse wave that travels green) and yellow (the complementary color of blue) also along the length of the rope. We can think of this wave as an tend to be prominent in thin soap films and layers of oil. The analog for a light wave, with the displacement of the rope at thickness of the film may vary slightly from point to point, any point corresponding to the magnitude and direction of causing other colors to destructively interfere and other the electric field. (The magnetic field, although an essential complementary colors to appear as a result. This gives the component of the electromagnetic wave, is not present in film a rainbow-like appearance F( igure 34). this analogy.) In this case, the wave is polarized along the By viewing an oil or gasoline slick from different angles, vertical axis in which you are moving your arm. you will see the colors shift even though the film itself has We described in Section I how electromagnetic waves are not changed thickness. This is because the light reaching produced when a charged particle, such as an electron, your eyes is incident at a different angle, and therefore undergoes acceleration. An electron that accelerates

2018–2019 Science35 Resource Guide along a vertical axis will produce vertically polarized light, allowing light that is polarized perpendicular to this axis whereas an electron that accelerates along a horizontal to pass through (Figure 35). What would you expect to axis will produce horizontally polarized light. Linearly see if you stacked two polarizing filters, oriented them polarized light, also known as plane polarized light, is perpendicular to one another, and then looked through made up of waves that are polarized along a single axis. them? No light would pass through both filters, so they At every point in a linearly polarized wave, the electric field would appear completely dark. is always directed parallel to the axis of polarization. Light reflected from nonmetallic surfaces is partially Unpolarized light is made up of waves that are polarized polarized by the surface. The amount of polarization along many different axes. Most everyday light sources, depends on the angle of incidence. Most of the reflected such as light bulbs, candles, and the Sun, emit unpolarized light is polarized parallel to the surface from which light. Light from these sources is produced by a multitude it is reflected. There is one angle of incidence, called of electrons that are vibrating in all different directions, so the polarizing angle, at which reflected light will there is no preferred direction of polarization. How, then, be completely polarized parallel to the surface of the can we obtain polarized light? The most common method is interface. The polarizing angle depends on the index to use a specially prepared material to separate out light of of refraction of both materials on either side of the a specific polarization from unpolarized light. reflective interface. The polarizing angle is also known as Brewster’s angle after Sir David Brewster, a Scottish In 1938, E. H. Land discovered a material that could physicist who first discovered it. polarize light, which he called the Polaroid. A common Polaroid filter is made up of many molecules called The polarization of reflected light is a common effect that hydrocarbons arranged in long parallel strands. The plays a role in our everyday lives. Light that is reflected material is prepared such that the constituent molecules from surfaces such as water, snow, or glass undergoes are especially good conductors, and they tend to absorb some degree of polarization. Polarized sunglasses tend to light that is polarized along their axis of orientation while have vertically-oriented polarization axes because most

FIGURE 35 Northwest Pa. Collegiate Academy - Erie, PA

A polarizer takes incident unpolarized light and transmits linearly polarized light.

2018–2019 Science36 Resource Guide of the light they are designed to block is reflected from placed within the focal length of the lens and viewed from horizontal surfaces and therefore tends to be horizontally the opposite side. The resulting image is virtual, magnified, polarized. Likewise, photographers often use polarizing upright, and appears farther away than the object actually is. filters to reduce unwanted glare from reflective surfaces such as water or glass. MICROSCOPES A single lens can only provide so much magnification of small objects. In order to probe finer details at small scales, OPTICAL INSTRUMENTS lenses can be combined in order to increase their overall Lenses can be combined in various ways to form images magnifying power, as in a microscope. In general, when that we would not be able to see with our eyes alone. considering the behavior of light in multi-lens systems, we In the remainder of this section, we will consider three can treat the image produced by one lens as the object for familiar examples of optical instruments: simple magnifiers, the other lens. A compound microscope consists of two microscopes, and telescopes. Each of these instruments can converging lenses; the lens closer to the object is called be understood by applying similar optical principles. the objective lens and the lens closer to the observer is called the eyepiece (Figure 37). The object to be observed SIMPLE MAGNIFIER is placed just outside the focal length of the objective lens. The simple magnifier is among the most basic optical The purpose of the objective lens is to form a real, inverted instruments, as it consists of just a single converging lens image at a point within the focal length of the eyepiece. (Figure 36). If you’ve ever used a magnifying glass, you are The eyepiece then acts as a simple magnifier for this familiar with the ability of a simple magnifier to produce intermediate image. The final image is virtual, magnified, enlarged images of small objects such as newsprint or an and inverted. insect. In order for magnification to occur, the object must be The inventor of the compound microscope is unknown,

FIGURE 36 Northwest Pa. Collegiate Academy - Erie, PA

A simple magnifier consists of a single converging lens. The solid vertical arrow is the object, and the dotted vertical arrow is the resulting virtual image.

2018–2019 Science37 Resource Guide although the first models were demonstrated around 1621. which employs a 2.4-meter concave mirror, was deployed Many seventeenth-century scientists, including Galileo, in 1990 from Space Shuttle Discovery and remains in experimented with multi-lens systems for the purposes of operation today. The Keck Observatory on Mauna Kea magnification. Although our ability to precisely manufacture in Hawaii features two reflecting telescopes with mirror lenses has steadily improved the magnification power of diameters of 10 meters, placing them among the largest compound microscopes over several centuries, there is a optical telescopes in the world today. fundamental limit to how powerful such microscopes can be. This limit is governed not by the strength of the lenses but A refracting telescope uses a combination of lenses by the nature of light itself; in order to be seen, the object to form an image of the distant object. Like a microscope, being viewed under a microscope must be at least as large refracting telescopes use a converging objective lens that as a wavelength of light (i.e., 400–700 nm). For smaller is nearer to the object and an eyepiece through which the length scales, electron microscopes are a more appropriate observer views the final image. Because the object to be instrument because they operate with smaller wavelengths. viewed is so far away, light rays reach the objective lens nearly parallel to one another and are focused to form TELESCOPES a real, inverted image essentially at the focal point of the Telescopes are used to view very distant objects such objective lens (Figure 38). By positioning the eyepiece so that as planets, stars, nebulae, and galaxies. A reflecting this intermediate image is just inside its focal point, it then acts telescope uses a concave mirror to collect and focus light as a simple magnifier to produce the final virtual image. The from very distant objects such as planets, moons, and stars. length of a refracting telescope is essentially the sum of the Isaac Newton is credited with assembling the first reflecting focal lengths of the objective lens and the eyepiece. telescope in 1688. Since Newton’s time, reflecting Refracting telescopes are useful for viewing larger, brighter telescopes have been instrumental in countless astronomical objects in the night sky such as the moon and planets. Some discoveries, including the discovery of the planet Uranus by astronomical objects, such as galaxies, are so distant that William Herschel in 1781. The Hubble Space Telescope, a large diameter telescope is necessary to collect enough

FIGURE 37 Northwest Pa. Collegiate Academy - Erie, PA

A ray diagram of a compound microscope.

2018–2019 Science38 Resource Guide FIGURE 38

A ray diagram of a refracting telescope. light for proper viewing. Manufacturing the large lenses slightly dependent on wavelength. Consequently, that would be necessary for a refracting telescope is different colors of light undergo different amounts difficult and costly; large-diameter lenses are also heavy of refraction, an effect known as dispersion. and prone to sagging. Furthermore, as we have discussed Dispersion causes the spectrum of colors produced previously, refracting telescopes are more susceptible to by prisms and observed in rainbows. chromatic aberration than reflecting telescopes that use ±± Total internal reflection is an optical phenomenon concave mirrors to focus light. For these reasons, reflecting in which a light ray traveling from a region of telescopes tend to be more useful for the purposes of higher refractive index to a region of lower astronomical research. refractive index is completely reflected at the boundary. Total internal reflection is the basis for SECTION II SUMMARY fiber-optic technology. A mirage is a naturally occurring optical illusion Light travels in a straight line until it encounters ±± ±± caused by light refracting through air of different a boundary with another medium. The ray

densities. A commonly observed mirage is the Northwest Pa. Collegiate Academy - Erie, PA approximation is a simplification that allows us illusion of pools of water on a hot roadway. to represent light as rays, or straight lines with a direction. ±± Real images are formed by light rays passing through the location of the image. Virtual images are The law of reflection states that the angle of ±± not formed by light rays meeting; they are images incidence equals the angle of reflection. Reflection from which light rays appear to be diverging. from a smooth surface is called specular reflection; reflection from a rough surface is called diffuse ±± Curved mirrors produce images with different reflection. characteristics than plane mirrors. Convex mirrors are curved outward toward incoming light and Refraction is the bending of light as it moves ±± produce only virtual images. Concave mirrors between transparent materials with different are curved inward away from incoming light and indices of refraction. The index of refraction of a produce real or virtual images depending on the material is equal to the speed of light in a vacuum location of the object. divided by the speed of light in that material. ± A lens is a shaped piece of transparent material that ±± The speed of light in a transparent material is also ±

2018–2019 Science39 Resource Guide refracts light rays to produce an image. Converging overlapping waves are in phase; their amplitudes lenses refract parallel light rays so that they meet at combine to produce a wave with a greater a focal point. Diverging lenses refract parallel light amplitude. Destructive interference occurs when rays so that they spread apart from a focal point. two out of phase waves combine to cancel each ±± We can use a ray diagram to predict where a lens other out. will produce an image of an object. By tracing ±± The iridescent colors seen in a soap bubble or rays that emerge from a point object and using the oil slick are caused by an effect called thin-film properties of lenses, we can find the image point interference. Light that is transmitted through a thin where the rays converge (or in the case of a virtual transparent film may be reflected back up and image, where the rays appear to be diverging from). interfere with light that was reflected from the top ±± No lens or mirror produces a perfect image. of the film. Spherical aberration occurs when light passing ±± Linearly polarized light is made up of waves that through the edges of a lens is focused at a are polarized along a single axis. Unpolarized light slightly different point than light passing near the can be polarized after passing through a polarizing center. Chromatic aberration is a consequence of material, such as Polaroid, or after reflecting from a dispersion. surface such as the ocean or a roadway. Brewster’s ±± In the seventeenth century, Christiaan Huygens angle is a special angle of incidence at which was an early proponent of the wave theory of light reflected from a surface will be completely light. According to Huygens’ principle, light waves polarized parallel to the surface. spread outward from a source like ripples in water. ±± Multiple lenses can be combined, as in a ±± Traveling waves can meet and pass through one microscope or telescope, to form images that we another without affecting or destroying either wave. would not be able see with our eyes alone. A According to the principle of superposition, we can compound microscope uses two converging lenses add the amplitude of each wave together point-by- to a produce a magnified image of a small nearby point to find the amplitude of the resulting wave. object. A refracting telescope uses two converging lenses to produce an image of a distant object. ±± Constructive interference occurs when two Northwest Pa. Collegiate Academy - Erie, PA

2018–2019 Science40 Resource Guide Section III

Lasers

INTRODUCTION LIGHT EMISSION AND The first part of this section will cover the basic building ABSORPTION blocks of laser operation, beginning with how light is emitted by atoms. From there, we will describe the processes of THE ATOM incandescence, fluorescence, and phosphorescence, all In order to understand how light is emitted, we must first of which are relevant to laser operation. The section will consider the structure and properties of the atom. Atoms conclude with an overview of how lasers work and a history are the building blocks of the world around us. Atoms contain of the development of the laser. a nucleus, which is a dense core that comprises most of the atom’s mass, and an outer region inhabited by bound

FIGURE 39 Northwest Pa. Collegiate Academy - Erie, PA

A neutral atom contains an equal number of protons and electrons. You can combine protons and electrons to form atoms using the “Build an Atom” PhET app at phet.colorado.edu.

2018–2019 Science41 Resource Guide electrons. The nucleus is made up of positively charged particles called protons as well as neutrons, which have no FIGURE 40 electric charge. The nucleus is surrounded by negatively charged electrons. These electrons occupy “shells” that have different spatial distributions. Some are spherical, some are barbell-shaped, and others have more complex distributions. Due to the attractive force between oppositely charged particles, the farther an electron is from the nucleus, the more potential energy it has with respect to the nucleus.

A substance composed of only one kind of atom is called an element. Each element in the periodic table has a characteristic atomic number, which is equal to the number of protons in the nucleus of an atom of that element. Electrons and protons have the same magnitude, but opposite sign, of electric charge. Because atoms are electrically neutral, they must have the same number of electrons as protons (Figure 39). (Atoms can gain or lose electrons to become ions, which have an overall negative or positive charge.) Hydrogen, which has an atomic number of 1, contains a single proton in its nucleus, orbited by a single electron. Likewise, all carbon atoms (atomic number 6) contain six protons and six electrons. QUANTIZATION Lasers are fundamentally quantum mechanical devices. That is, the operation of a laser is fully dependent on the quantum nature of light and matter. Let’s explain what this means. Before 1900, scientists assumed energy could vary continuously and be endlessly subdivided. No experiment had provided evidence to contradict this viewpoint. In Max Planck proposed the quantum hypothesis that 1900, a German physicist named Max Planck (Figure energy is made up of discrete units. 40) proposed that the energy could only be emitted or absorbed in discrete bundles that are multiples of a PHOTONS fundamental unit, or quantum, of energy. This relationship Experiments conducted around the same time as Planck’s can be expressed as hypothesis revealed that light itself exhibits similar quantum Northwest Pa. Collegiate Academy - Erie, PA E = nhf , behavior. Heinrich Hertz had discovered in 1887 that metals illuminated with ultraviolet light tended to produce where h = 6.63 × 10−34 J s is called Planck’s constant; f sparks. J. J. Thomson later determined that these sparks is the frequency of oscillation; and n is a positive integer. were actually electrons being ejected from the surface That is, energy could be exchanged⋅ in amounts of hf, of the metal. This phenomenon became known as the 2hf, 3hf, etc., but not anything in between. Because the photoelectric effect. Physicists initially attempted to energy can only take certain specific values, we say that it use Maxwell’s electromagnetic wave model to explain is “quantized.” As an analogy, imagine dunes of sand that the photoelectric effect. They reasoned that since appear smooth from a distance, but are in fact coarse and electromagnetic fields exert forces on charged particles, grainy when observed close up. Planck’s constant is small the oscillating fields in a light wave could theoretically push enough that we do not notice the quantization of energy in an electron within an atom back and forth as one might our everyday experience—the quantized packets of energy push a child on a swing. Eventually, the electron could are too small to be individually perceived by our senses. have enough energy to escape the atom, and the metal, altogether.

2018–2019 Science42 Resource Guide FIGURE 41

The photoelectric effect. Light below a certain frequency does not eject electrons, whereas higher-frequency light ejects electrons with more energy. These observations cannot be explained by treating light as a wave.

However, further investigation of the photoelectric effect a lower-frequency beam of light. Einstein was awarded in 1902 resulted in several observations that contradicted the Nobel Prize in 1921, in part for his explanation of the physicists’ expectations under the wave model. For photoelectric effect. instance, electrons were emitted almost instantaneously (less than a nanosecond) after the metal was illuminated, Photons are packets of energy transferred from the whereas the wave model predicted a time delay. electromagnetic field. Although they have zero mass, Furthermore, light below a certain frequency was not photons have both energy and momentum. It takes an observed to eject any electrons, regardless of the intensity extremely large number of photons to make up most of the of the light. According to the existing theory, any frequency radiation we experience on a daily basis because the energy of light would be expected to eject electrons if the incident of any individual photon in the visible spectrum is so small. light was sufficiently intense. In addition, increasing the For example, a quick estimate can be made that a single 100 21 frequency of the incident light resulted in higher-energy watt light bulb will release on the order of 10 photons every second. Normally we do not “feel” an individual photon any

electrons being emitted—however, there should have been Northwest Pa. Collegiate Academy - Erie, PA no relationship between light frequency and electron more than we would feel an individual droplet of water while energy when treating light as a wave (Figure 41). swimming through the ocean.

Drawing from Planck’s theory, in 1905 Albert Einstein Before we move on, we should address an important proposed that light itself is quantized into discrete energy point. Although they seem contradictory, the photon, or packets that exhibit properties of particles. Einstein argued particle, nature of light does not invalidate the wave nature that the photoelectric effect could only be explained as of light we discussed in the previous section. Light exhibits packets of light colliding with electrons, thereby transferring properties of both waves and particles, depending on their energy and ejecting the electrons from the metal. the experiment (Figure 42). This seemingly contradictory These packets of light energy eventually became known behavior is known as wave-particle duality. The as photons. Furthermore, Einstein argued that the energy photoelectric effect is a demonstration of the particle of these photons obeys Planck’s relation E = hf, which was nature of light, whereas double-slit interference is a wave consistent with experimental results from the photoelectric phenomenon. Wave-particle duality is a puzzling aspect of effect. Thus, an individual photon from a higher-frequency the physical world that took the world’s leading physicists beam of light carries more energy than a photon from many years to accept.

2018–2019 Science43 Resource Guide FIGURE 42

A double slit illuminated with extremely weak laser light, so that photons effectively pass through one by one. Each tiny dot is a single photon detection. Notice how an interference pattern is gradually built up over time, illustrating the dual nature of light.

From Weis and Dimitrova.9

THE BOHR MODEL with more than one electron, the energy levels follow In 1913, Niels Bohr proposed a model of the hydrogen more complicated rules, and additional quantum numbers atom that incorporated the quantum aspects of Planck’s are required to specify the energy state of an electron. hypothesis. Bohr adapted the existing planetary model Physicists associate these quantum numbers with quantities of the atom, in which electrons revolved around a central describing specific characteristics of the electron, including nucleus, but added a key assumption: Bohr suggested that the shell, the subshell, position in a shell, and spin. the electron’s energy was only allowed to take on particular Fortunately, the details of these quantum states are not values and could not be anything in between those values. necessary to obtain a basic understanding of lasers, so we Furthermore, these energy levels correspond to specific won’t cover them here. fixed orbits around the nucleus F( igure 43). A staircase According to an important rule called the Pauli exclusion serves as a useful analogy for the energy levels in an atom. principle, each electron in an atom must occupy a unique Just as it’s not possible for an object to reside in between quantum state. Equivalently, each electron in an atom two steps, an electron cannot occupy a state between two must have a unique set of quantum numbers. Shells and energy levels. subshells usually are identified by a number and letter, such Bohr indexed the energy levels of the hydrogen atom as 1s or 2p. The number refers to the principal quantum according to a quantum number that could take the number, the same as Bohr’s original quantum number whole number values = 1, 2, 3, and so on. The lowest n. The letter identifies sub-shells formed under complex

n Northwest Pa. Collegiate Academy - Erie, PA allowable energy level, for which n = 1, is known as the quantum-mechanical rules. Each shell can contain two or ground state of the atom. In this state, the electron is more electrons, but each electron must have a unique set also closest to the nucleus. Energy levels that are higher of quantum numbers. In the simplest case of two-electron than the ground state are called excited states. Electrons shells, the two electrons spin in opposite directions. must accept energy, for example by absorbing a photon, in If atoms are in their ground state, electrons fill in energy order to move from one energy level to a higher one. Only levels from the lowest-energy or innermost shell until the certain frequencies of photons can be absorbed by an number of electrons equals the number of protons in the individual atomic system, and therefore each energy level nucleus. The primary quantum number of the last ground- diagram is quantized by the allowable transitions between state electron corresponds to the row the element occupies states, which is unique for each type of atom. on the periodic table. This “stacking” property of electrons Hydrogen is the simplest atom in the universe due to its helps explain some of the chemical properties observed single orbital electron. As a result, physicists often use within families of elements in the periodic table. For it as the standard model to explain the basic rules of instance, the noble gases helium, argon, neon, krypton, quantum mechanics and atomic energy levels. In atoms and xenon all have filled electron shells and therefore do

2018–2019 Science44 Resource Guide FIGURE 43

The Bohr model of the hydrogen atom includes fixed orbitals (left) with corresponding energy levels (right). Only the first three levels are shown. not react or combine easily with other elements. In contrast, levels. The frequency of the emitted light is given by the alkali metals such as lithium and sodium have an unpaired Planck relation E = hf (Figure 44). electron and tend to be highly reactive. A neon light is a familiar example of a gas-discharge There are many more types of energy levels in addition to lamp, which produces light as a result of atomic de- those found in solitary atoms. Molecules have electronic excitation (Figure 45). Gas-discharge lamps consist of glass energy levels of their own, which become more complex as tubes filled with a noble gas such as argon, neon, or xenon. the number of electrons and atoms in the molecule increases. The color of the lamp depends on which gas is contained

Molecules also have energy levels that depend on vibrations within the tube. True neon lights are reddish-orange in Northwest Pa. Collegiate Academy - Erie, PA of atoms within them and on the rotation of the entire color; other colors, such as green or blue, are produced molecule. All of these energy levels are quantized as well. by different gases. At each end of the tube are electrodes that are connected to a voltage source. The voltage causes ATOMIC EXCITATION AND EMISSION electrons to leave one of the electrodes and vibrate back Each element has a characteristic set of energy levels that and forth within the tube, colliding with millions of atoms as are common to every atom of that element. The process they do so. The collisions with the electrons transfer energy of an electron moving from a lower to a higher energy to the atoms, exciting their orbital electrons to higher level is called excitation, which occurs when the atom energy levels. The atoms emit a characteristic photon as absorbs energy corresponding to a particular energy they de-excite, and the process continues. level transition. This energy input can result from absorbing a photon, but can also come from kinetic energy due to City street lights commonly use a form of gas-discharge collisions with other particles or thermal energy from being lamp to produce light. Street lights containing mercury heated. Once they are in an excited state, electrons can vapor produce light with intense blue and violet drop to lower energy levels by emitting a photon with components, giving them a different appearance than white energy equal to the difference in energy between the two light from an incandescent light bulb. Many street lights

2018–2019 Science45 Resource Guide FIGURE 44

An atom emits a photon when an excited electron drops to a lower energy level. The energy of the photon is equal to the difference in energy between the two levels.

now use low-pressure sodium vapor that consumes less for chemical analysis and the discovery of new elements in Northwest Pa. Collegiate Academy - Erie, PA energy; these lights are primarily yellow in color. While the mid-nineteenth century. walking along a city street at night, you may be able to tell what type of gas is contained within the street light Aurora borealis and aurora australis, also known as the based on the color of light it produces. Using a prism or Northern and Southern Lights, are spectacular visual diffraction grating, you can actually separate and identify phenomena that can be explained by atomic excitation. the individual frequencies that each lamp emits. The Sun emits a continual stream of charged particles that become trapped in Earth’s magnetic field and spiral down Certain elements produce brilliant colors of light when through the atmosphere. These particles collide with atoms ignited, which is another result of atomic de-excitation. and molecules in the atmosphere, thereby exciting them to For example, table salt produces a bright yellow flame, higher energy levels. Oxygen atoms emit a pale green color, because a common transition between energy levels in nitrogen molecules produce red-violet light, and nitrogen sodium corresponds to yellow light. Likewise, copper ions emit blue-violet light. In addition to visible light, auroras produces a bluish-green flame, and lithium burns bright also emit infrared, ultraviolet, and X-ray radiation. magenta red. These characteristic colors served as a basis

2018–2019 Science46 Resource Guide FIGURE 45

Neon lights are a form of gas-discharge lamp, which produces light as a result of atomic de-excitation.

EMISSION SPECTRA outside the visible spectrum include the Lyman series (which The characteristic set of wavelengths emitted by a collection involves transitions to n = 1) and the Paschen series (n = 3). of excited atoms is called an emission spectrum. The more electrons an atom contains, the more complicated Emission spectra can be viewed with a measurement device the energy level structure becomes. The hydrogen spectral called a spectroscope, which uses a prism or grating lines are relatively simple, containing only a few visible lines; to separate light emitted by a collection of excited atoms this is because hydrogen contains only a single electron. into component wavelengths. Figure 46 shows a common Northwest Pa. Collegiate Academy - Erie, PA For atoms with larger atomic numbers, the electrons interact configuration for a spectroscope, in which the emitted light with one another and with the nucleus, creating many more is passed through a thin slit and projected onto a viewing energy levels and thus many more possible transitions. Iron, screen. Each component color is refracted to a definite which has an atomic number of 26, contains 26 electrons. position on the screen, forming a distinct image of the slit Notice that iron has many more visible spectral lines than as a narrow line. These differently colored lines are often hydrogen due to its more complex electron configuration. called spectral lines. Since each element has a characteristic set of energy levels, Figure 47 shows emission spectra for an incandescent the emission spectrum is also unique to each element. source, a hydrogen lamp, and a collection of excited iron We can therefore think of these spectral lines as a sort atoms. The emission spectrum for hydrogen contains four of “fingerprint” that indicates the presence of a specific distinct lines in the visible spectrum, which occur as a result element. The process of analyzing spectral lines to identify of a hydrogen atom transitioning to the n = 2 state from a the chemical makeup of an excited sample is called higher energy level. This set of emission lines is known as atomic spectroscopy. Atomic spectroscopy came into its the Balmer series. Other sets of hydrogen emission lines own as a chemical analysis technique in the late 1800s.

2018–2019 Science47 Resource Guide FIGURE 46 Try It! Make a Spectroscope

You don’t need special equipment in order to observe spectral lines for yourself. You can use a spare CD and a cardboard paper towel tube to assemble your own spectroscope. Detailed instructions and pictures can be found at the Exploratorium website.10 The CD acts as a diffraction grating to spread out the individual wavelengths of visible light received by a radiant object. Test out your spectroscope by viewing an incandescent bulb and a fluorescent light. Try observing a variety of light sources of various colors, including city street lights, neon lights, LEDs, and incandescent sources. (Caution: You should not use the spectroscope to view light sources that would be unsafe to view with your eye alone, such as the Sun or laser light.) What differences do you observe between the emission spectra for these light sources, and what does it tell you about how they emit light?

INCANDESCENCE All matter that has a temperature above 0 K (“absolute zero”) radiates electromagnetic waves, which is known as thermal radiation. At room temperature, objects radiate mostly infrared waves, which cannot be detected by our eyes but can be captured by an infrared-sensitive camera. When objects become sufficiently hot (around 800 K), A basic spectroscope apparatus using a prism and a they begin to glow with a visible color dependent on their narrow slit in cardboard (top). Notice the difference in temperature. Familiar examples include a red-hot stove spectral lines from an incandescent bulb (middle) and burner or a yellow-white light bulb filament F( igure 48). a fluorescent bulb (bottom). Incandescence is the production of light due to an object Northwest Pa. Collegiate Academy - Erie, PA © Timwether / Wikimedia Commons / CC-BY-SA-3.0 being at a high temperature.

Unlike light from a neon tube, thermal radiation is emitted The probability that a particular transition will occur as a continuous range of wavelengths instead of a discrete depends on the population of the two states and their set of spectral lines. Does this mean the tungsten atoms in quantum characteristics. Certain types of quantum a light bulb filament contain an infinite number of energy transitions are far more likely than others, so each transition levels? No, it doesn’t—if we were to vaporize the tungsten to a lower energy state has its own characteristic lifetime, atoms and excite them, they too would emit a characteristic or average length of time for an electron to undergo set of discrete spectral lines. So, what’s going on? a transition between those two states. As we will see, these lifetimes are an important factor in laser operation. When in the gas phase, atoms are separated by large Complex interactions between adjacent atoms in liquids distances relative to their size and thus have a low or solids blur energy levels so that simple absorption probability of interacting with one another. The process of and emission lines are not visible. This creates significant electron excitation and de-excitation within one atom will not differences between gas and solid-state lasers. be affected by the presence of other atoms within the gas.

2018–2019 Science48 Resource Guide FIGURE 47

The emission spectra for an incandescent source, hydrogen, and iron.

transitions. Consequently, atoms in the solid phase tend to FIGURE 48 emit photons with a continuous range of frequencies.

The frequencies of light emitted through incandescence are dependent on the temperature of the object. Figure 49 shows several intensity distribution curves for objects at various temperatures. Notice that the intensity of radiation is not evenly distributed across wavelengths; for any given temperature, there is a “peak” wavelength that is emitted with the greatest intensity. Furthermore, experimental evidence has shown that the peak wavelength is inversely proportional to the temperature of the object. 1

Northwest Pa. Collegiate Academy - Erie, PA peak T Accordingly, if the kelvin temperature흀 ∝ of an incandescent object is doubled, the peak wavelength of emitted electromagnetic waves will be halved. This relationship allows us to easily compare the surface temperature of A hot stove coil glows red due to incandescence. A distant stars based on their distributions of emitted light. For large fraction of energy is also being radiated as instance, since the wavelength of violet light is just about infrared waves, which we perceive as heat. half that of red light, we can estimate that a violet-hot star © Joel Kramer / flickr / CC-BY-SA-2.0 has approximately twice the surface temperature of a red-hot star. As an object is heated from room temperature, In contrast, atoms in the solid phase are bunched together it first glows “red-hot” as the peak wavelength shifts into and may even share electrons in the case of metals. Energy the visible spectrum from the infrared. Eventually, as the levels of closely-packed atoms effectively overlap, allowing distribution peak overlaps the entire visible spectrum, the electrons to undergo a much greater number of possible object will appear “white hot.”

2018–2019 Science49 Resource Guide ABSORPTION SPECTRA in to the emission spectrum of sunlight we find that it is not Electrons can move to higher energy levels by absorbing perfectly continuous—instead, there are thin lines where electromagnetic radiation. An absorption spectrum is certain frequencies of light are missing. These absorption created by passing a continuous spectrum of electromagnetic lines are known as Fraunhofer lines after Joseph von waves through a collection of atoms. What emerges from Fraunhofer who first discovered them and determined the atoms is a continuous spectrum except for black lines, their wavelengths. Fraunhofer lines occur because the called absorption lines, where specific wavelengths of light light emitted from the body of a star is absorbed by the have been absorbed. Figure 50 shows a segment of the atmosphere of cooler gases that surround it. In 1868, emission and absorption spectra for hydrogen. Notice that scientists deduced that a set of then-unknown spectral lines the positions of absorption lines correspond with the emission from the Sun belonged to a yet undiscovered element, lines for the same element. which they called helium. Thus, solar atomic spectroscopy revealed helium to be a new element almost three decades The Sun is an incandescent source of light, yet if we zoom before it was first found on Earth.

FIGURE 49 Northwest Pa. Collegiate Academy - Erie, PA

Intensity distributions of emitted electromagnetic waves for objects at 3000 K, 4000 K, and 5000 K. Notice how the peak wavelength becomes shorter as the temperature of the object increases.

2018–2019 Science50 Resource Guide Spectral lines from stars also provide evidence of how fast red shift in their spectral lines, which serves as evidence that they are moving relative to us. If you’ve ever stood near a the universe is expanding. train as it passes, you may have experienced how the sound of the whistle seems to become lower in pitch at the moment FLUORESCENCE the train moves past your position. Although the frequency Different de-excitation pathways can cause atoms to emit of the whistle has not changed, the sound waves that reach different combination of photons. Fluorescence occurs your ears are shifted lower in frequency. This frequency shift when a material absorbs a photon and then de-excites due to a moving source, known as the Doppler effect, by emitting a photon with a lower frequency (i.e., energy). also applies to electromagnetic waves. Because we know Returning to the staircase analogy we explored earlier, the exact frequencies of spectral lines due to measurements imagine leaping to the top of a small staircase in a single here on Earth, we can compare them with spectral lines from jump. You could get back to the bottom by stepping down distant objects to determine the relative shift. If the spectral each lower step one by one, or by skipping over a step lines are shifted toward the red side of the spectrum (i.e., and then taking a smaller step. Likewise, an atom excited lower in frequency), we know the star or galaxy is moving by high-frequency (and thus higher-energy) ultraviolet light away from Earth. Nearly all galaxies we observe exhibit a can take smaller steps to a lower energy state by emitting

FIGURE 50 Northwest Pa. Collegiate Academy - Erie, PA

Emission spectrum (top) and absorption spectrum (bottom) for hydrogen.

2018–2019 Science51 Resource Guide FIGURE 51 FIGURE 52

A collection of fluorescent minerals illuminated with ultraviolet light.

of different compositions. Each mineral has a different crystal structure made up of a particular combination of elements, giving them each a distinct set of energy levels. Like the fluorescent ink on a black light poster, the atoms in the mineral structures are excited by high-energy ultraviolet photons. As the excited electrons cascade down An energy level diagram of fluorescence. After to lower energy levels, they emit photons with frequencies absorbing a photon, an atom emits a photon with a corresponding to the tiny energy-level spacings between different frequency by dropping down two energy each step. Every excited atom emits a characteristic set levels in a row. of frequencies, and no two different minerals emit light of exactly the same color. lower-frequency (and thus less-energetic) visible light as it returns to the ground state (Figure 51). This property will PHOSPHORESCENCE become important later in explaining laser operation. Atoms in excited states normally have lifetimes on the order of nanoseconds; that is, they undergo de-excitation You may be familiar with fluorescent posters that glow vibrant Northwest Pa. Collegiate Academy - Erie, PA within about 10−9 seconds of being excited. However, due colors when exposed to an artificial source of ultraviolet light to various quantum mechanical effects, certain excited (sometimes known as a “blacklight”). Fluorescent dyes are states can have lifetimes on the order of microseconds or used in paints and fabrics to make them glow when they are even milliseconds. Energy states that exhibit this degree of bombarded with ultraviolet photons in sunlight. Fluorescence relative stability are said to be metastable. Metastable has also been introduced as an anti-counterfeiting security states exist because certain electron transitions are feature on paper currency around the world, including in less probable than others under the rules of quantum the United States. If you shine ultraviolet light on a $10, $20, mechanics. Electrons that are excited to metastable states $50, or $100 bill printed within the past decade, you will effectively become “stuck” because the de-excitation reveal a fluorescent thread that cannot otherwise be seen process occurs at a relatively low rate. Materials with this with visible light alone. property are said to exhibit phosphorescence. An example of natural fluorescence is the brilliant response You’re already familiar with the phenomenon of of various minerals to ultraviolet light (Figure 52). Many phosphorescence if you’ve ever observed “glow in the natural science museums have dedicated exhibits to dark” paint, toys, stickers, or light switches. Many such showcase the wide array of colors produced by minerals

2018–2019 Science52 Resource Guide a small amount of a halogen gas, such as iodine, that slows FIGURE 53 the rate of evaporation of the tungsten filament and extends the lifetime of the bulb. Halogen lamps operate at a higher temperature and are slightly more energy efficient than standard incandescent bulbs.

Although they have been ubiquitous for more than a century, incandescent bulbs are not particularly efficient sources of light. A standard incandescent light bulb converts less than 5 percent of the energy it consumes into visible light—the rest is emitted as invisible frequencies of electromagnetic waves, which we may perceive as heat. Accordingly, incandescent bulbs are in the process of being phased out around the world in favor of more energy- efficient light sources. Some countries, including Mexico, have banned them entirely. Fluorescent Lamps and CFLs A fluorescent lamp is a long, cylindrical glass tube filled with a low-pressure mercury vapor. Fluorescent lamps operate similarly to gas-discharge lamps. Electrodes connected to a voltage source discharge energetic This bird figurine glows in the dark due to electrons into the tube, where they collide with mercury phosphorescence. atoms and excite them to a higher energy level. The excited mercury atoms emit photons primarily with ultraviolet materials can be made to emit light by first exposing them frequencies. The emitted ultraviolet light interacts with to daylight. The visible light excites the atoms within the phosphors, powdery materials coating the inside of the material to a higher energy level, where they remain in a tube. Upon absorbing an ultraviolet photon, the phosphors metastable state while gradually undergoing de-excitation. fluoresce, thereby emitting an assortment of low-frequency The persistent glow is caused by atoms emitting photons photons that collectively we perceive as white light. A during the de-excitation process. Unlike fluorescent compact fluorescent lamp (CFL) is a small fluorescent tube materials, which stop glowing immediately after the lamp that is twisted into a helical shape and attached to external source of radiation is turned off, phosphorescent a screw-in base like that of an incandescent bulb. CFLs materials can glow for seconds, minutes, or even hours are more energy efficient than incandescent bulbs and after no longer being illuminated (Figure 53). have average lifetimes around ten times longer. Different phosphors can be used to coat the inside of the tube in Northwest Pa. Collegiate Academy - Erie, PA LAMPS order to produce different shades of white light (Figure 54). Incandescent Lamps The primary disadvantage of fluorescent lamps, both An incandescent lamp, or incandescent light bulb, consists compact and regular, is that they pose a significant of a glass globe enclosing a thin tungsten filament. An environmental and health hazard due to their mercury electric current heats the filament to a temperature between content. Fluorescent bulbs must be disposed of properly so 2000 K and 3300 K, at which point it emits a continuous as not to release poisonous mercury into the surrounding air spectrum of light with an intensity peak in the infrared. or water. While CFL bulbs were promoted for a few years Argon gas is commonly used to fill incandescent light as an energy-efficient alternative to incandescent bulbs, bulbs; if oxygen reached the hot tungsten filament, it would the lowering cost of LED (light-emitting diode) bulbs in undergo rapid oxidation and be destroyed. Over time, the recent years has made them a preferred candidate for safe filament gradually evaporates until it breaks and no longer energy-efficient lighting. In early 2016, General Electric conducts electricity—at this point, the bulb has “burned announced that they would be phasing out CFL production out.” A halogen lamp is an incandescent lamp that includes in the United States in favor of LED bulbs.11

2018–2019 Science53 Resource Guide FIGURE 54

Comparison of an incandescent light bulb (left) and five CFL bulbs. They differ slightly in color due to different types of phosphor coating inside the tubes.

Light-Emitting Diodes the two semiconductor layers, the electrons will be energetic enough to cross the barrier and fill the holes in A diode is an electronic device that allows electric charge the adjoining layer. In doing so, the electrons lose energy in to pass through it in only one direction. Diodes serve many the same manner as atomic electrons dropping to a lower different functions in a variety of electronic devices we rely on energy level. This energy is released in the form of a photon every day. For example, diodes are commonly used in power of visible light (Figure 55). supplies to convert alternating current (AC) to direct current Northwest Pa. Collegiate Academy - Erie, PA (DC). A photodiode is another type of diode that produces The color of the emitted light is determined by the depth an electric current when light is incident upon it, which is of the energy “holes,” which is a property of the elements essential to the operation of solar cells. Light-emitting used to create the semiconductor layers. A larger drop in diodes (LEDs) work like photodiodes in reverse; they emit energy will correspond to a higher energy photon (i.e., light when an electric current is passed through them. more blue) whereas a lower drop will correspond to a lower energy photon (i.e., more red). The first light-emitting An LED consists of a junction between two semiconductor diodes (LEDs) were developed in the 1960s and were only layers. A semiconductor is a material that can be made capable of emitting red light. In the 1990s, LEDs of many to conduct electricity under some conditions but not others. different colors became widely available. One semiconductor layer (the “n-type”) contains excess electrons, whereas the other layer (the “p-type”) contains Today, LEDs are used in a wide variety of lighting “holes” where electrons could be accepted. A barrier applications in commercial, industrial, and residential prevents the electrons from moving across the boundary. settings. LED arrays are commonly found in traffic lights, If a voltage (from a battery, for instance) is placed across automobile brake lights, and electronic billboards and

2018–2019 Science54 Resource Guide displays. Within the last few years, LED light bulbs have LASERS supplanted CFLs as the consensus solution for energy- efficient lighting in most applications. LED bulbs have long LASER OPERATION lifetimes—approximately five times that of a CFL bulb, and A laser is a device that emits a narrow beam of single- forty times that of an incandescent bulb. Furthermore, LEDs wavelength, coherent light as a result of stimulated consume less energy than other bulb designs and do not emission. The term laser began as an acronym for “Light have disposal risks. LEDs also do not undergo abrupt failure Amplification by Stimulated Emission of Radiation.” The as incandescent bulbs do; instead they very gradually output power of laser light can vary from a few thousandths decrease in brightness over many hours of use. of a watt (in the case of laser pointers) to several thousand watts (industrial laser cutters). Every laser contains an

FIGURE 55 Northwest Pa. Collegiate Academy - Erie, PA

A light-emitting diode emits photons when excess electrons from an n-type semiconductor layer fill holes in a p-type semiconductor layer.

2018–2019 Science55 Resource Guide active medium, which is the source of atoms that will Stimulated Emission undergo cycles of excitation and de-excitation to release photons that will form the laser beam. Depending on the We have previously discussed how an atom can become type of laser, the active medium can be a solid, liquid, or excited from a lower to a higher energy level by absorbing gas. Let’s walk through the necessary conditions for laser a photon with energy equal to the difference between the operation, all of which build upon the principles of energy energies of two levels. Eventually, the atom will return to a levels, excitation, fluorescence, and phosphorescence that lower energy state by emitting a photon in a process known we covered earlier in this section. as spontaneous emission. Although we can predict on average how long it will take for de-excitation to occur (i.e.,

FIGURE 56

Stimulated emission occurs when a photon triggers a de-excitation event.

FIGURE 57 Northwest Pa. Collegiate Academy - Erie, PA

A population inversion exists when more atoms are in an excited state than in the ground state.

2018–2019 Science56 Resource Guide the lifetime of the state), it’s impossible to predict exactly Population Inversion when a specific excited atom will spontaneously emit a photon. Furthermore, the direction of the emitted photon is A functional laser depends on not just a handful of also random. stimulated emission events, but rather a full cascade that will continually generate a stream of stimulated emission Atoms can also transition to a lower energy level through a photons. When a collection of atoms is in thermodynamic process called stimulated emission. While an atom is in equilibrium (that is, it is not exchanging energy with an excited state, the oscillating electric field from a passing its surroundings), the vast majority of the atoms tend photon (at the same frequency as—or very close to—the to be in the lowest possible energy state. This poses a transition frequency of the excited electron) can cause the problem because an emitted photon is almost certain to atom to emit a second photon that oscillates in precise be absorbed by an atom in the ground state instead of synchrony with the first F( igure 56). Most significantly, stimulating emission in an excited atom. the emitted photon is identical in frequency, phase, and direction to the first photon. These identical photons are Clearly, in order to sustain the chain reaction of stimulated what make up a laser beam. emission events, the atoms must be prepared such that more are in an excited state than are in the ground state. This

FIGURE 58 Northwest Pa. Collegiate Academy - Erie, PA

A simplified energy diagram of a three-level laser. The pump source excites the atoms to the highest energy level, where they are likely to drop down to the metastable state. Because the lifetime of the metastable state is relatively long, stimulated emission is likely to occur before the electrons undergo spontaneous emission.

2018–2019 Science57 Resource Guide FIGURE 59

An optical resonator is essential to amplify emitted light waves while allowing a fraction of the photons to escape, forming the output beam.

condition is known as population inversion (Figure 57). level, where they quickly de-excite to the metastable level. Stimulated emission will continue as long as population The metastable level is chosen to have a lifetime generally inversion exists within the active medium, but it will slow a thousand times longer than that of the higher energy down and eventually stop if a majority of the atoms are no level. This process accumulates a large population of atoms longer in the higher-energy state. in the metastable state, thereby establishing a population inversion between it and the ground state. Energy Levels A laser is not itself a source of energy; rather, it needs Achieving population inversion and sustaining a chain energy from an external source in order to continually of stimulated emission events requires a configuration of maintain population inversion. An external source of energy levels with specific characteristics. If the excited energy, called the pump source, excites the atoms in the atoms undergo spontaneous emission before stimulated active medium to the metastable state. Although a variety emission can take place, there will not be a sustained beam of techniques have been used in laser pumping, the most of identical photons traveling in the same direction. For common methods are optical pumping and electrical Northwest Pa. Collegiate Academy - Erie, PA this reason, the excited state must be metastable to ensure pumping. Optical pumping is the use of an intense light that atoms will remain in that state long enough to sustain source, such as a flashlamp, arc lamp, or external laser to a population inversion. Some amount of spontaneous excite atoms through photon absorption. Electrical pumping emission is unavoidable, but the longer the lifetime of the involves the use of an electric discharge or current to cause state, the easier it is to ensure that stimulated emission will atomic excitation. dominate. Three-level lasers are not an optimal solution for laser Although two-level lasers are theoretically possible under operation; a more efficient method involves four energy certain conditions, it is generally unfeasible to excite atoms levels. A four-level laser adds an additional energy level directly into a metastable state. A more practical approach above the ground state, which becomes the lower level for involves three energy levels: a ground state, an excited state the laser emission transition. In other words, the lower level with a short lifetime (relative to the other two transitions), of the laser transition is not the ground state, which makes and a metastable state with a slightly lower energy (Figure it easier to maintain a population inversion between the 58). Atoms are excited, or “pumped,” into the higher energy laser transition states. In a four-level laser, the lower level of

2018–2019 Science58 Resource Guide the laser transition starts out nearly empty because it has a the laser beam. One mirror is coated to be completely higher energy than the ground state. Thus, only exciting a reflective (100 percent chance of reflection) whereas small fraction of the ground state atoms to the metastable the other is coated to be partially reflective (~95 percent state is sufficient to establish a population inversion. chance of reflection). Photons that “leak” from the partially reflective mirror form the laser beam. It’s possible for light to Optical Cavity reflect back and forth several hundred times before exiting the resonator (Figure 59). Once population inversion is achieved (i.e., most of the atoms are in an excited state), a single atom undergoing PROPERTIES OF LASER LIGHT de-excitation emits a photon that causes a chain reaction Laser light has several properties that make it useful for of stimulated emission events: that photon causes stimulated many practical applications. Laser light is monochromatic, emission in a nearby atom, which emits an identical photon, directional, and coherent. By comparison, ordinary white which stimulates the emission of another photon, and so on. light is a combination of many wavelengths of light, emits In order to sustain the chain of stimulated emission events isotropically (in all directions), and is a mixture of many out‐ and amplify the laser beam, the active medium must be of‐phase wavelengths. These three properties of laser light surrounded by an optical cavity (also known as a laser are what can make it more hazardous than ordinary light— cavity or optical resonator). laser light is capable of depositing a lot of energy within The simplest optical cavity consists of two parallel mirrors, a small area. We will discuss each of these properties in one of which is slightly transmitting to allow the output of greater detail.

FIGURE 60 Northwest Pa. Collegiate Academy - Erie, PA

Now that you’ve got the basics of laser operation down, try building your own laser with the “Lasers” PhET app at phet.colorado.edu. You can configure the energy levels, cavity reflectivity, and wavelength. How powerful can you make your laser?

2018–2019 Science59 Resource Guide Monochromatic FIGURE 61 Because the photons emitted by a laser all correspond to the same energy transition, they all have the same frequency. Single-frequency light such as this is often described as monochromatic. In comparison, recall that thermal radiation (such as that produced by an incandescent light source) produces a continuous spectrum of frequencies with different intensities.

Laser light is not perfectly monochromatic; there is some “spread” due to Doppler shifts from the motion of atoms or molecules within the active medium. The line width or bandwidth of a laser describes the spread of its spectrum of emitted frequencies. However, this spread is extremely narrow compared to the spectrum of frequencies emitted Coherent waves (top) and incoherent waves (bottom). by, say, an incandescent light bulb. Each individual wave has the same frequency.

Directional waves have random phases. Even a collection of waves The light from a laser emerges as a very narrow beam with with identical frequencies can be incoherent if they are not very little divergence, or spread. We often refer to a beam in-phase relative to one another. LEDs, for example, emit with this property as being collimated. If you’ve used a light that is single-frequency but not coherent. Coherence in laser pointer while giving a presentation or distracting a laser light is a direct consequence of stimulated emission. feline companion, you are familiar with the ability of lasers Coherence in laser light is important for observing to project a point of light, even from a relatively large interference effects, which has important applications in distance. A laser’s high degree of collimation is a direct precision measurement. Interferometry is the use of result of the precise alignment of the parallel mirrors that superimposed waves to make extremely fine measurements form the optical cavity. As the light waves reflect back and of small displacements, surface irregularities, or changes in forth many times within the cavity, the mirrors constrain refractive index. A basic interferometer uses a beam splitter the waves to an axis perpendicular to the surfaces of both and mirrors to overlap beams of light from a coherent source mirrors. Any light that is slightly “off-axis” will be lost from such as a laser. By slightly adjusting the path length difference the cavity and thus will never form part of the final beam. between the two beams, shifts in the observed interference The highly collimated nature of a laser beam makes it pattern will occur as the relative phases of the two beams highly useful but also highly dangerous. You should never change. This technique allows measurements to be made on look directly into a laser beam because the highly parallel the length scale of the wavelength of light being used. Northwest Pa. Collegiate Academy - Erie, PA rays can focus to a nearly microscopic dot on the retina of your eye, causing almost instant damage to the retina. On TYPES OF LASERS the other hand, the ability of lasers to focus so precisely Gas Lasers contributes to their wide range of both medical and Gas lasers use a low-pressure gas mixture as an active industrial applications. In medicine, lasers can be used as medium. Most gas lasers are excited by passing an electric sharp scalpels; in industry, they can serve as fast, powerful, current through the gas, delivered by electrodes placed and computer-controllable cutting tools. at opposite ends of the tube. The helium-neon laser (or Coherent HeNe laser) is a common gas laser that produces light in the visible spectrum at a wavelength of 632.8 nm. HeNe Light that is made up of waves that are “in-phase” relative laser cavities contain a mixture of helium and neon gas. to one another is said to be coherent. In other words, The helium atoms are excited by an applied current and the peaks and troughs of the waves exactly align (Figure then collide with neon atoms to excite them to the state that 61). An ordinary light source, such as an incandescent causes the 632.8 nm radiation. The bright red output and light bulb, produces light that is incoherent, meaning the relatively low cost of HeNe lasers make them well suited for

2018–2019 Science60 Resource Guide applications. For example, HeNe lasers were originally FIGURE 62 used in grocery store checkout scanners, but have largely been replaced by laser diodes for this purpose. Solid-State Lasers Solid-state lasers use an active medium consisting of a solid crystalline or glass rod (known as the host) containing light-emitting atoms (the active species). The first laser ever built was a solid-state laser using synthetic ruby, which is corundum (aluminum oxide crystal) with chromium as an active species. In most solid-state laser materials, the active species is identified first (typically by its chemical symbol), followed by the host material. For example, the Ti:sapphire laser consists of titanium atoms in sapphire crystal. Both the active species and the host are important in solid-state lasers. The active species determines the laser transition, but its interactions with the host may shift the wavelength slightly.

Neodymium (Nd) is commonly used as an active species in solid-state laser crystals. For example, the Nd:YAG (neodymium-yttrium aluminum garnet) is one of the most common types of laser, with applications in research, A helium-neon laser. medicine, manufacturing, and other fields F( igure 63).

© David Monniaux / Wikimedia Commons / CC-BY-SA-3.0 Nd:YAG lasers typically emit infrared light with a wavelength of 1064 nm although other wavelengths are possible. Other many low-power applications in educational and research host materials for neodymium include YLF (yttrium lithium fluoride) and glass. The host material is selected based on its laboratories (Figure 62). optical, thermal, and mechanical properties. Other examples of gas lasers include the carbon dioxide

(CO2) laser, which is a high-efficiency laser that operates in the infrared band. Carbon dioxide lasers are commonly FIGURE 63 used for high-power industrial applications such as welding and cutting.

Excimer lasers are a type of gas laser that rely on the excitation of “dimer” molecules, such as argon fluoride, Northwest Pa. Collegiate Academy - Erie, PA that are stable only in the excited state. Excimer lasers were first demonstrated in the mid-1970s and are capable of removing extremely fine layers of surface material by breaking molecular bonds without burning or heating the surrounding area. For this reason, excimer lasers are well- suited for precision etching of plastics or semiconductor circuits, as well as delicate eye surgery such as LASIK.

An advantage of gas lasers over other laser types is that the gas medium tends to be both relatively inexpensive and largely resistant to damage. However, gas lasers are also typically larger than other types due to the low density of the medium. In recent years, gas lasers have seen a A frequency-doubled 1064 nm Nd:YAG laser. decline in sales as they have gradually been replaced by © Kkmurray / Wikimedia Commons / CC-BY-SA-3.0 solid-state and semiconductor lasers for many commercial

2018–2019 Science61 Resource Guide Semiconductor Diode Lasers render it non-functional. Furthermore, the delicate nature of the semiconductors makes laser diodes more sensitive Semiconductor diode lasers, commonly known as laser to static discharges and currents. Excess electrical current diodes, operate using the same basic principles as light- can cause the diode to become inoperable. Laser diodes emitting diodes, but with some important differences. Like can also degrade in power efficiency over time, gradually an LED, a laser diode consists of two semiconductor layers, requiring more power to output the same beam intensity. an n-type with an excess of electrons and a p-type with electron “holes” to be filled. The semiconductor layers are separated by a microscopic region called the active HISTORY OF THE LASER layer that serves as the optical resonator. Laser diodes operate at much higher currents than LEDs, typically 1900–50: EARLY FOUNDATIONS The origins of the laser can be traced back to an idea from around ten times greater. Whereas an LED emits photons Albert Einstein in the formative years of quantum theory. in all directions from its junction layer, a laser diode is Having emerged as an early visionary in twentieth-century configured with reflective ends to form an optical resonator physics with his 1905 explanation of the photoelectric in the region between the semiconductor layers. In a laser effect, Einstein continued to make contributions to the theory diode, stimulated emission occurs when a photon emitted by one electron transition triggers another electron to fill a of light emission (Figure 65). In 1917, Einstein published a hole, and so on, resulting in a coherent beam of light that paper entitled Zur Quantentheorie der Strahlung (“On the emerges from one side of the diode. Quantum Theory of Radiation”) in which he proposed the concept of stimulated emission. Laser diodes are compact and easy to mass-produce. In terms of sheer numbers, they are the most common type FIGURE 65 of laser. Their small size makes them well suited for use in low-power applications such as laser pointers, laser printers, and CD/DVD players (Figure 64). Laser diodes can also be operated at lower voltages than other types of lasers. While gas and solid-state lasers require input voltages on the order of kilovolts, laser diodes can be operated at only a few volts.

Laser diodes are typically not as collimated as beams from other types of lasers. In many cases, an external lens is used to correct the shape of the beam, which contributes to the overall fragility of the laser since damage to the lens could

FIGURE 64 Northwest Pa. Collegiate Academy - Erie, PA

Albert Einstein originally proposed the concept of A laser diode, with a penny for scale. stimulated emission.

2018–2019 Science62 Resource Guide In 1947, Willis Lamb and R.C. Retherford were the first to experimentally demonstrate stimulated emission. In 1950, FIGURE 66 French physicist Alfred Kastler proposed the technique of optical pumping, whereby a collection of atoms could be excited to a higher energy state through absorption of incident light. This idea would later have tremendous implications for laser operation, and Kastler was awarded the 1966 Nobel Prize in Physics for this contribution.

EARLY 1950s: CHARLES TOWNES AND THE MASER By 1950, stimulated emission seemed to be an interesting phenomenon with few practical applications due to the difficulty of achieving the necessary population inversion. That would change following a breakthrough from Charles H. Townes. During World War II, Townes had worked to develop radar systems that operated at microwave frequencies. After the war, Townes applied his expertise in microwaves to the field of molecular spectroscopy, using microwaves to study the structure of molecules and atoms. Townes joined the faculty at Columbia University in 1948 and became a professor in 1950.

In 1951, Townes was sitting on a park bench in Washington, Charles Townes standing next to the ammonia maser, D.C., when he devised a method for producing stimulated the precursor to the laser. emission at microwave frequencies. Townes’s idea was to a pass a beam of excited ammonia molecules through population inversion in a three-level quantum system. Basov a reflective cavity, where they would emit microwaves. and Prokhorov’s three-level model correctly anticipated the The cavity would contain and amplify the waves as they configuration that would ultimately be used in the first laser. were emitted, thereby establishing a stimulated emission condition. A small hole in one side of the cavity would LATE 1950s: LASER CONCEIVED permit the coherent beam of microwaves to escape. Townes Once the maser had been firmly established, a natural dubbed his device the “maser,” which stood for “Microwave question emerged: could the same principle of stimulated Amplification by Stimulated Emission of Radiation.” emission be extended to visible wavelengths of light? In Eminent physicists of the time, including Niels Bohr and 1957, Charles Townes and Arthur Schawlow explored the Isidor Rabi, expressed skepticism that Townes’ maser idea possibility of infrared light amplifiers but eventually moved on Northwest Pa. Collegiate Academy - Erie, PA was practical or even possible.12 Nevertheless, Townes to visible wavelengths, which they called “optical masers.” began work on the maser with two graduate students at Townes partnered with a Columbia graduate student named Columbia named James P. Gordon and Herbert J. Zeiger. In Gordon Gould who was interested in optical pumping. 1954, Townes and his team demonstrated the first working On November 13, 1957, Gould wrote an entry in his lab maser using excited ammonia molecules in accordance notebook wherein he used the term “laser,” which is believed with Townes’ original vision. Townes’ maser produced to be the first usage of the term F( igure 67). about 10 nanowatts of power at a wavelength around 1 As the possibility of developing a working laser seemed cm (Figure 66). closer than ever, many scientists around the world While Townes was developing the maser, Soviet physicists recognized the groundbreaking possibilities presented Nikolay Basov and Aleksandr Prokhorov were trying to by the device. Gordon Gould filed a patent application model stimulated emission in quantum systems with more in April 1959, but it was rejected by the U.S. Patent than two energy levels. In 1955, Basov and Prokhorov Office. Instead, the patent was awarded to Schawlow suggested that optical pumping could be used to create a and Townes on behalf of Bell Labs, who had applied nine

2018–2019 Science63 Resource Guide Laboratories also tried using ruby, but faced challenges FIGURE 67 with generating enough energy to achieve population inversion. Ali Javan, a former doctoral student of Townes at Columbia, attempted a gas laser using a helium-neon mixture.

On May 16, 1960, Theodore Maiman succeeded in demonstrating the first functional laser at Hughes Research Laboratories in California. Maiman’s laser used a synthetic ruby crystal the size of a fingertip to produce 694 nm red laser light. Although other research teams had also been experimenting with ruby as an active medium, Maiman’s innovation was to use a flashlamp to deliver an intense optical pulse that would excite a sufficient number of atoms (Figure 68). Although Maiman’s laser could only operate in brief, intense pulses, it was the first demonstration of laser operation.

Maiman’s discovery set off a flurry of activity among the other research teams. Careful analysis of the technique led to the development of several other laser types less than a year later. Shortly after Maiman’s demonstration, Arthur Schawlow succeeded in producing laser light using a different kind of ruby crystal. In December 1960, Ali Javan, and William R. Bennett, Jr., and Donald R. Herriott succeeded in demonstrating the first gas laser, the HeNe, at Bell Labs. In 1962, Robert Hall demonstrated the first diode laser using gallium arsenide (GaAs) as the semiconductor Gordon Gould’s 1957 notebook entry in which he material. Other laser types would debut in the 1960s, including the CO and Nd:YAG lasers in 1964; and the coined the term “LASER.” 2 tunable dye laser in 1967. months earlier. Gould sued, and after thirty years of legal The 1964 Nobel Prize in Physics was awarded jointly to proceedings he was awarded forty-eight patents, including Charles Townes, Nikolay Basov, and Aleksandr Prokhorov optical pumping and a variety of laser applications. The “for fundamental work in the field of quantum electronics, development of the laser, which was a product of many which has led to the construction of oscillators and 13 theoretical and experimental breakthroughs over the course amplifiers based on the maser-laser principle.” Half was Northwest Pa. Collegiate Academy - Erie, PA of several decades, serves as an example of how difficult it awarded to Townes; the remaining half was shared by can be to assign credit for scientific innovations. Basov and Prokhorov.

1960s: THE RACE TO BUILD THE LASER 1970s–PRESENT Following a 1958 publication by Townes and Schawlow During the 1970s, laser technology began to enter our daily confirming that masers operating at visible wavelengths lives with a steady stream of practical applications. On June were a theoretical possibility, all that remained was for 26, 1974, a pack of chewing gum became the first product a research team to demonstrate the first working version. ever scanned by a laser barcode reader. In 1978, LaserDisc Several teams across the United States joined the race technology, which used HeNe lasers to read optical discs, to construct the first laser. At Columbia, Townes began became commercially available for the first time. Although work on a potassium gas laser, since its energy levels LaserDiscs did not succeed commercially in the long-run, the were well understood. Arthur Schawlow explored the use same technology led to the introduction of audio CD players of ruby crystals at Bell Laboratories, since they were in in 1982. The first commercial laser printer was released by ample supply. Irwin Wieder at Westinghouse Research IBM in 1976, and in 1984 laser printing entered mainstream

2018–2019 Science64 Resource Guide FIGURE 68

Diagram of a ruby laser. availability with Hewlett-Packard’s introduction of the became known as photons. The energy of a desktop-size HP Laserjet. In the next section, we will cover a photon is directly proportional to its frequency. much wider assortment of applications of laser technology. ±± Electrons occupy quantum states within atoms, corresponding to energy levels that are quantized. The lowest energy level is known as the ground SECTION III SUMMARY Northwest Pa. Collegiate Academy - Erie, PA state; higher energy levels are called excited states. Atoms are the building blocks of the world around ±± When atoms become excited, electrons move to us. Each element has a characteristic atomic number, ±± higher energy levels. An electron can transition which is equal to the number of protons in the to a lower energy level by emitting a photon with nucleus of an atom of that element. Neutral atoms energy equal to the difference in energy between contain equal numbers of protons and electrons. the two levels. Lasers rely on the quantum nature of light and ±± The characteristic set of wavelengths emitted matter. In 1900, Max Planck proposed that energy ±± by a collection of excited atoms is called an can only be exchanged in discrete bundles that emission spectrum. Since every element has a are multiples of a fundamental unit, or quantum, of unique electron configuration, the collection of energy. wavelengths emitted by an element is also unique. In 1905, Albert Einstein proposed that light itself is ±± Incandescence is the production of light due to an quantized into discrete energy packets that exhibit ±± object being at a high temperature. Incandescent properties of particles. These packets eventually objects emit a continuous spectrum of wavelengths.

2018–2019 Science65 Resource Guide The peak wavelength depends on the object’s coherent. In order to sustain a chain of stimulated temperature. emission events, a population inversion must be ±± Fluorescence occurs when a material absorbs a maintained between a metastable state and a photon and then de-excites by emitting a photon lower-energy state. with a lower frequency. Phosphorescence is a ±± Although they all rely on stimulated emission and related process that takes place over a longer time optical resonators, gas lasers, solid-state lasers, scale because the de-excitation occurs from a and laser diodes all use different types of materials metastable state, which has a longer lifetime. as active media. Consequently, each type of laser ±± Incandescent light bulbs, fluorescent lights, has advantages and disadvantages that make it and light-emitting diodes all rely on different suitable for a different set of applications. mechanisms to produce light. As a result, they have ±± In 1954, Charles Townes invented the maser, the different energy efficiencies and emit light with precursor to the laser that operated at microwave different properties. frequencies. Theodore Maiman developed the first ±± Lasers use stimulated emission of atoms to produce working laser, a solid-state ruby laser, in 1960. light that is monochromatic, directional, and Northwest Pa. Collegiate Academy - Erie, PA

2018–2019 Science66 Resource Guide Section IV

Applications of Lasers

INTRODUCTION by timing when the sensor measured absorption. In the described configuration involving a Ti:Sapphire laser Lasers are used for a variety of applications in scientific, with a 30-second scan at a uniform rate, a detection of medical, industrial, and commercial fields. In this section, absorption 20.37 seconds after the start of the sweep we will describe some of the most significant applications of would indicate an absorption wavelength of 760.37 nm. lasers in these categories, as well as some of the scientific Tunable semiconductor lasers can be swept across that figures who developed these applications. As we will see, wavelength range even faster. the output power and wavelength of a laser determines the kind of applications for which it is most appropriate. Infrared absorption spectroscopy, involving wavelengths of a few micrometers, is a reliable technique for identifying organic compounds. This is because molecules are SCIENTIFIC APPLICATIONS commonly identified by their patterns of vibration and rotation, and the frequencies of infrared light match the LASER SPECTROSCOPY vibrational and rotational frequencies of most molecules, In the previous section, we described how elements can be resulting in absorption. Laser sources are available in the identified by their characteristic absorption and emission infrared region, but non-laser sources are easier and less spectra; this analysis technique is known as spectroscopy. costly to use when the extremely high resolution of laser Laser spectroscopy is a branch of spectroscopy in sources is not essential. which a laser is used to illuminate the sample being studied in order to determine a precise absorption spectrum. CONFOCAL LASER SCANNING The 1981 Nobel Prize in Physics was awarded in part MICROSCOPY to Nicolaas Bloembergen and Arthur Schawlow for the Lasers can also be used as part of microscopes, although development of laser spectroscopy. Due to their high the manner in which an image is formed is very different degree of monochromaticity, lasers output a much narrower from a conventional microscope. Confocal laser band of wavelengths than conventional light sources, scanning microscopy, or CLSM, is a technique that

allowing researchers greater precision in identifying Northwest Pa. Collegiate Academy - Erie, PA reconstructs a three-dimensional rendition of the specimen. the precise wavelengths that have been absorbed. First, a laser beam is focused onto a single point on a For example, an ordinary light source could show that two-dimensional “slice” of the sample to be imaged. absorption occurs around 750 nm for a given sample. A Mirrors are then used to scan the laser back and forth laser, on the other hand, could show that light was in fact everywhere along that two-dimensional plane. As the laser absorbed at two wavelengths in that range: 749.87 and scans, molecules illuminated by the laser light undergo 750.13 nm. Furthermore, lasers can also identify absorption fluorescence and emit light that is refocused through a lines too weak to see using other techniques. pinhole aperture to be collected by a sensor. An amplifier Laser absorption spectroscopy is most often performed is then used to convert the intensity of the emitted light into using tunable lasers, which can be “swept” across a an electrical signal (Figure 70). The information collected particular range of emitted wavelengths. For example, an about each section of the object is then used to create the absorption experiment might involve a titanium-sapphire three-dimensional model of the specimen. laser that sweeps from 740 to 770 nm once every thirty The basic principle of confocal scanning microscopy was seconds (Figure 69). If the wavelength varies uniformly with first developed by Marvin Minsky, a Harvard postdoctoral time, the precise absorption wavelength can be determined

2018–2019 Science67 Resource Guide FIGURE 69

A schematic illustrating laser absorption spectroscopy with a tunable laser. I0 is the reference intensity of the laser, and I is the signal intensity after passing through the sample. The signal intensity is reduced by an amount ∆I relative to the reference, indicating absorption at that wavelength. student, in the mid-1950s. However, Minsky’s technique speeds, advances in digital storage technology, and the initially had limited practical utility because it required development of more stable and powerful lasers have Northwest Pa. Collegiate Academy - Erie, PA a high-intensity light source and substantial information significantly widened the applicability of CLSM as a processing power. The development of the laser in the next research tool. Today, CLSM is used to conduct routine decade brought renewed interest in confocal scanning investigations on molecules, cells, and living tissues that microscopy. A functioning confocal laser scanning were not possible even a few years ago. You can learn microscope was first demonstrated in 1969 by M. David more about CLSM using the scanning microscope simulator Egger and Paul Davidovits at Yale. Egger would go on at the Olympus Microscopy Resource Center.14 to publish the first recognizable images of cells in 1973. Advances in both computer and laser technology in the MANIPULATING TINY OBJECTS following decades, coupled with new algorithms for digital Optical Tweezers manipulation of images, led to a growing interest in confocal microscopy. The first commercially available instruments Laser light is not only useful for probing matter at microscopic utilizing this technique began to appear in 1987. length scales, it can also be used to confine matter for experimental purposes. In 1970, Arthur Ashkin at Bell Since the 1990s, improvements in computer processing Laboratories first reported on the ability of laser light to exert

2018–2019 Science68 Resource Guide FIGURE 70

A confocal laser scanning microscope. Northwest Pa. Collegiate Academy - Erie, PA forces on microscopic particles. This discovery led to the in momentum of the photon. According to momentum development of optical tweezers, which are scientific conservation, the object acquires a change in momentum instruments that use a tightly focused beam of laser light to that is equal and opposite to that of the photon. By hold microscopic particles stable in three dimensions. The precisely focusing the beam of photons on the object to be beam can then be used to move these particles around, constrained, these continual momentum “kicks” effectively similar to the way a pair of tweezers can be used to push the object inward toward the center of the beam. manipulate small objects. The optical tweezing effect was demonstrated by Ashkin and other colleagues in 1986. Optical tweezers have been used to sort cells, track the movement of bacteria, and measure piconewton-scale Optical tweezers rely on the fact that photons have forces on microscopic particles. momentum. As we discussed earlier in the guide, when light is transmitted through an object, it refracts and changes A primary application of optical tweezers is studying the direction. This change in direction corresponds to a change properties of DNA. A strand of DNA can be attached to a glass or polystyrene bead that is then “gripped” by the

2018–2019 Science69 Resource Guide FIGURE 71

Use optical tweezers to virtually manipulate an individual DNA strand using the “Optical Tweezers and Applications” PhET app at phet.colorado.edu. optical tweezers. By pulling on the bead, scientists have measurement of how much kinetic energy, on average, measured the elasticity of DNA as well as the amount of its constituent particles have. At room temperature, atoms force needed to break bonds within a DNA molecule. DNA and molecules in the air move at an average velocity of molecules within cells are wrapped around proteins but around 500 meters per second. Naturally, it is difficult to are unwound during the process of replication. In 2001, conduct experiments—and measure effects due to quantum Northwest Pa. Collegiate Academy - Erie, PA researchers at Cornell used optical tweezers to unwind mechanics—on individual atoms moving at this speed. a strand of DNA from a protein and discovered that the molecule spontaneously rewrapped to its original state The basics of laser cooling can be understood through a once released. The researchers concluded that enzymes combination of quantum and classical principles. When within the cell must act as “molecular motors” that are an atom absorbs a photon, it “recoils” in the direction the capable of exerting forces on the DNA strand. photon was moving due to conservation of momentum. The excited atom will then undergo spontaneous emission, Laser Cooling releasing a photon in a random direction. After many thousands of these absorption and emission processes, the Lasers can also be used to control and slow down a atoms will tend to be pushed away from the laser beam. collection of atoms in a technique known as laser We can therefore think of the laser as exerting an overall cooling, for which Claude Cohen-Tannoudji, Bill “pressure” on the gas. Phillips, and Steven Chu were awarded the 1997 Nobel Prize in Physics. The temperature of any substance is a In order to “cool” the atoms, physicists use a technique called Doppler cooling to selectively slow atoms moving

2018–2019 Science70 Resource Guide FIGURE 72

(a) Schematic of a laser cooling apparatus (b) A fluorescence image of laser-cooled cesium atoms. Based on the intensity of fluorescence, an estimated 1.5-2 billion atoms are contained within the cloud.

toward the laser. According to the Doppler effect, the frequencies of atomic transitions are extremely stable and apparent frequency of any wave is shifted due to relative are identical across all atoms of the same element. motion between the source and receiver. If you are moving toward a source of light, the frequency you observe will be Since 1967, the second has been defined as the unit of slightly higher in frequency than what you would observe time that elapses during exactly 9,192,631,770 oscillations at rest. Therefore, by setting the cooling laser frequency of the radiation produced by transitions between the two to slightly below an energy transition, only atoms moving hyperfine levels of the cesium-133 ground state. (Hyperfine toward the laser will be excited by the light and thus slowed splitting results from an extremely small shift in ground state down. A conventional atom trap uses six separate laser energy due to magnetic interaction between the electron beams directed inward toward a vacuum chamber in order spin and nuclear spin.) Atomic clocks have been created to cool a collection of atoms (Figure 72). using various elements including hydrogen, rubidium, mercury, strontium, and aluminum. Cesium is the current How cold can we make a collection of atoms? As laser standard used by the National Institute of Standards cooling techniques have improved, physicists have been and Technology (NIST), which regulates standards of able to cool atoms to temperatures progressively closer to measurement in the United States. Northwest Pa. Collegiate Academy - Erie, PA absolute zero. In 2015, a Stanford research lab reported cooling a rubidium gas to 50 trillionths of a Kelvin.15 The The second is the most accurately known unit in the SI average speed of a rubidium atom in this gas would be less system of measurement. The first cesium atomic clock, than 70 thousandths of a millimeter per second. created in 1955, was accurate to within one second in 300 years (Figure 73). Today, cesium clocks have achieved Atomic Clocks accuracy to within one second every 300 million years.16 The substantial increase in accuracy can be attributed Our ability to control atoms using methods such as laser to ongoing advances in laser cooling and spectroscopy cooling has led to substantial advances in precision techniques, which have improved our ability to accurately timekeeping and standardization. Early clocks measured measure frequencies and reduce experimental noise in time through primarily mechanical means, such as the atomic systems. swinging of a pendulum. These devices tend to be sensitive to small environmental variations or energy dissipation NUCLEAR FUSION through friction or air resistance, making them unsuitable Extremely high-power lasers are being used to initiate as a standard for precision timekeeping. In contrast, the nuclear fusion reactions as part of research efforts into fusion

2018–2019 Science71 Resource Guide FIGURE 73

The first cesium atomic clock. as a viable power source. Nuclear fusion is a type of temperatures of at least ten million Kelvin, and temperatures nuclear reaction in which two smaller atomic nuclei combine of that magnitude are difficult to achieve and maintain in a to form a larger nucleus. (Nuclear fusion can be thought of laboratory or power plant. The possibility of laser-induced as the opposite of nuclear fission, in which a heavy nucleus fusion was first suggested around 1962, not long after the splits into smaller nuclei.) Fusion reactions involving light invention of the laser itself. The idea behind laser-induced nuclei, such as hydrogen, release energy. Chains of these fusion is to heat and compress hydrogen isotopes to the fusion reactions serve as the power source for stars, including high temperatures and pressures needed for their nuclei to Northwest Pa. Collegiate Academy - Erie, PA the Sun. An important fusion reaction is the combination of undergo fusion as they do in stars. This is done by focusing deuterium (a hydrogen nucleus consisting of a proton and very short and very high-energy laser pulses onto a target a neutron) and tritium (a hydrogen nucleus consisting of a made of fusion fuel, thereby heating and compressing the proton and two neutrons) (Figure 74). target to very high pressures and temperatures for a fleeting instant. Inertial forces implode the fusion targets, so this Nuclear fusion is often considered the ultimate power source approach is called inertial confinement fusion. because it relies on an abundant fuel source (hydrogen), does not produce radioactive waste, and does not present a Inertial confinement fusion relies on high-intensity laser risk of the catastrophic “meltdown” associated with nuclear beams to heat and compress a tiny pellet that contains fission reactors. However, many challenges remain before a mixture of deuterium and tritium (Figure 75). The goal fusion can become a viable power source. The greatest of inertial confinement is to cause the fusion reaction to challenge to controlled nuclear fusion is the extremely high proceed fast enough that there is not enough time for temperatures necessary to ensure that nuclei are energetic nuclei to move apart. Laser energy is concentrated in enough to fuse. Fusion of hydrogen nuclei in the Sun requires short, intense pulses, and the light is distributed uniformly

2018–2019 Science72 Resource Guide FIGURE 74 FIGURE 75

The deuterium-tritium target pellet at the National Nuclear fusion. Ignition Facility. © Haasrm / Wikimedia Commons / CC-BY-SA-3.0 extracted from the fusion reaction than was used to trigger across the surface of the target, producing a symmetrical the reaction itself.17 Although progress is being made, fusion explosion that forces the target material inward while ignition remains an elusive target that may take at least heating it to high temperatures. The deuterium-tritium pellet another decade of further research to achieve. is quickly ionized into a plasma and heated to temperatures above 108 K. Fusion is achieved in under 10–9 s, or less than one billionth of a second. MEDICAL APPLICATIONS

Government scientists have built a series of increasingly LASER SURGERY larger and more powerful lasers, which have demonstrated Lasers have had a tremendous impact on medicine. Laser that laser fusion is possible but not easy. Each step paved surgery techniques are based upon an understanding the way for a bigger and more powerful laser. The U.S. of how laser light interacts with biological tissue, which Department of Energy began construction of the latest in the entirely depends on the wavelength of light being used sequence, the National Ignition Facility (NIF), in 1997 at the and the nature of the tissue. The most important component

Lawrence Livermore National Laboratory in California. (The of biological tissue is water, which absorbs strongly in Northwest Pa. Collegiate Academy - Erie, PA name comes from the goal of crossing a threshold called the infrared range of the electromagnetic spectrum. In fusion ignition, at which the fusion reaction becomes fact, water is such a strong absorber of infrared and self-sustaining.) The NIF was completed in March 2009 and accounts for so much of the bulk of soft tissue that it is a is currently among the largest and highest-energy inertial fair approximation to consider tissue as absorbing light confinement fusion devices in the world. as if it was water. Water absorbs about 80 percent of the

incident 10.6-µm wavelength of a CO2 laser in the first 20 The NIF occupies three connected buildings that together µm, corresponding to the surface of exposed skin or tissue. are 704 feet long, 403 feet wide, and 85 feet tall. The NIF Absorption is even stronger at shorter wavelengths of 3–6 uses the most powerful laser assembly ever constructed, µm, although it is not uniform through the infrared band. driven by a system of amplified neodymium-glass lasers, to ignite fusion in a hydrogen pellet 2 mm in diameter. In Carbon dioxide lasers are a common choice for laser 2012, the NIF conducted a laser pulse with a power of surgery because they are readily available and emit 10.6-

500 trillion watts—equal to 1,000 times the power used µm wavelength light that is easily absorbed by tissue. A CO2 by the entire United States at any instant in time. In 2013, laser beam focused on tissue at a sufficiently high intensity NIF achieved an important milestone—more energy was will cause cells to be vaporized due to energy absorption.

2018–2019 Science73 Resource Guide The absorption is so strong that only the upper layer of cells Carbon dioxide lasers at 10.6 µm cannot be used to cut is vaporized; these cells absorb virtually all of the light, so bone because bone contains less water than other tissue. the lower level of cells survives with little damage (Figure However, lasers emitting at wavelengths of 3 µm can cut 10). The lower layer suffers even less damage from a laser bone because bone contains some water that strongly emitting at a wavelength of 3 µm because water absorbs absorbs that wavelength. Surgeons generally use other that wavelength even more strongly. tools for cutting bone and for many other types of surgery, particularly general surgery on internal organs, where lasers Another advantage of the 10.6-µm CO2 laser wavelength have no particular advantages over conventional scalpels. is that it penetrates just deep enough into tissue to seal small blood vessels and stop bleeding. This cauterization Other laser wavelengths in the ultraviolet, visible, and near infrared have other advantages for certain types of effect makes the CO2 laser especially valuable for surgery in regions rich in blood vessels, such as the gums and the medical treatment. For example, lasers can be used to treat female reproductive tract, by giving the surgeon a tool to “port-wine stains,” which are a type of dark red birthmark remove thin layers of blood-rich tissue. This is particularly resulting from defective blood vessels near the surface of useful in the treatment of gum disease and of endometriosis, a the skin. In this case, the laser is selected to match the peak condition affecting several million American women. Carbon absorption of the blood vessels causing the blemish. As dioxide lasers also are used in a type of heart surgery that another example, the 193-nm wavelength of the argon- creates new paths for blood vessels in the heart, called fluoride excimer laser is strongly absorbed by the lens of transmyocardial revascularization. In fact, certain types of the human eye, making it ideal for refractive surgery. Recent surgeries were only made possible with the introduction of advances in laser medicine have been made possible by the laser because they involved incisions that resulted in a matching the laser wavelength and power level to specific high degree of bleeding with a conventional scalpel. treatment needs.

FIGURE 76 Northwest Pa. Collegiate Academy - Erie, PA

Penetration depths of laser light for various laser types. CO2 lasers are capable of destroying only the outermost layer of skin cells while leaving the deeper tissue unaffected.

2018–2019 Science74 Resource Guide Refractive Eye Surgery of using surgical lasers to correct vision by reshaping the cornea followed almost immediately (Figure 77). The most commonly advertised type of laser surgery today uses an argon-fluoride excimer laser to reshape the lens of The first type of laser refractive surgery, known as the eye to correct refractive defects that otherwise would photorefractive keratectomy (PRK), uses an excimer laser require people to wear corrective eyeglasses or contact to ablate material from the cornea after the top layer of the lenses. Refractive surgery procedures are used to correct cornea, known as the epithelium, has been chemically wiped both myopia (near-sightedness) and hyperopia (far- away. Concern over the effects of removing the epithelium sightedness) as well as astigmatism, which is blurred vision led to the development of LASIK (laser-assisted in-situ due to uneven focusing of light by the eye. The development keratomileusis), in which the epithelium is peeled back before of the excimer laser in 1970 paved the way for the use of the excimer laser ablates tissue from the underlying layer, lasers in refractive eye surgery. Researchers in the early then put back in place afterward (Figure 78). The LASIK 1980s discovered that 193-nm pulses from argon-fluoride procedure was approved in the United States in 1999; by excimer lasers could ablate, or dissolve, tissue very efficiently 2002 LASIK had already become the world’s most common from the cornea, the transparent front layer of the eye. As the elective procedure.18 Although LASIK is currently the most cornea provides much of the eye’s refractive power, the idea common form of refractive surgery, PRK is still in use and new variations are under development.

FIGURE 77 Refractive surgery techniques have become steadily more efficient and successful as our laser and computational technology have improved. Current systems can measure the refractive profile of the eye to compute the precise corrections

FIGURE 78 Northwest Pa. Collegiate Academy - Erie, PA

The LASIK surgical procedure involves peeling back LASIK refractive surgery procedure being performed the epithelium and then reshaping the cornea using at National Naval Medical Center Bethesda in 2007. light from an excimer laser.

2018–2019 Science75 Resource Guide required to compensate for the eye’s defects. However, some into the dermis layer of the skin. Laser tattoo removal complications remain, and accuracy of the correction often works by illuminating tattoos at a wavelength absorbed depends on the healing response of the eye. Some patients by the tattoo pigment, thereby breaking down the pigment require a second surgery to complete correction. molecules to bleach the tattoo. The effectiveness of the procedure depends on the ink; some tattoos almost LASER DERMATOLOGY disappear, but others fade only slightly. A successful laser Laser dermatology procedures achieved early success treatment will generally leave a slightly discolored spot in treating dark-red birthmarks called “port wine where a traditional tattoo used to be. Laser tattoo removal stains,” which often appear on the face or neck. These may be made easier due to a new family of encapsulated discolorations result from blood in networks of abnormal tattoo inks, which are designed for people who might have blood vessels just under the surface of the skin. Because second thoughts. The ink is contained within tiny transparent port wine stains tend to be spread over the surface of plastic spheres, which protect them from degrading in the the skin, they cannot be treated by conventional surgery, body. A single laser treatment splits open the spheres, and concealment with cosmetics was often the only dumping the dye into cells, which break down the solution. Dermatologists made early attempts to treat port molecules to bleach the color. wine stains using blue-green argon laser light, but these procedures were painful, worked only with dark blemishes, Laser skin resurfacing is another cosmetic procedure in and could cause scarring. These problems were reduced by which a carbon dioxide laser is scanned across aged switching to pulsed dye lasers tuned to emit near the peak areas, particularly the face, neck, and hands, to remove the absorption of hemoglobin in blood at about 580 nm (Figure surface layer. This process removes surface wrinkles and 79). At this wavelength, light absorption is concentrated in blemishes, thereby exposing a fresh layer of skin. Laser skin blood vessels, thereby avoiding skin blisters, and can be resurfacing requires a few weeks for the skin to heal and used with lighter birthmarks or on children. Lasers also are return to normal color. used for treating other skin blemishes, such as acne scars. Lasers can also be used for the purpose of hair removal, Tattoo removal is another familiar medical application of which is accomplished by directing near-infrared laser light lasers. Tattoos consist of colored ink that has been inserted at hair follicles. A pigment called melanin in the hair and

FIGURE 79 Northwest Pa. Collegiate Academy - Erie, PA

Relative absorption versus wavelength of laser light by hemoglobin, melanin, and water.

2018–2019 Science76 Resource Guide FIGURE 80

Absorption rates of various metals by wavelength. Higher values indicate that light of that wavelength is more readily absorbed by the metal. Northwest Pa. Collegiate Academy - Erie, PA follicle absorbs the light, heating and killing the follicle, and LASER DENTISTRY thus stopping hair growth in areas where hair is undesired. There are three primary applications of laser dentistry. The effectiveness of laser hair removal treatment depends Lasers can be used to “drill” teeth, using solid-state erbium on the contrast between skin and hair color; the darker lasers that emit 3-µm wavelength light. Although these the hair and the lighter the skin, the more effective the lasers cannot remove solid enamel well, they can remove treatment. Different lasers may be used for different skin decayed areas and prepare cavities for repair. More and hair combinations. The 755-nm output of a pulsed important to both dentist and patient, laser pulses are less alexandrite laser works well, but only on light skin. Arrays threatening than the traditional whirring mechanical drill of diode lasers emitting pulses at 810 nm are used for light and can be less painful as well. As mentioned earlier, to medium skin, and neodymium lasers emitting at 1064 nm carbon dioxide lasers can also be used to treat gum are used for darker skin. It is more difficult to remove light- disease by removing swollen tissue. Laser teeth whitening is colored hair than dark-colored hair with laser light due to another popular cosmetic dental treatment that uses visible the lack of melanin pigment. argon and neodymium lasers to activate a special gel solution that has been applied to the patient’s teeth.

2018–2019 Science77 Resource Guide INDUSTRIAL APPLICATIONS peak power may not reach high enough levels to vaporize and remove material from the hole.) The number of pulses Many industrial and manufacturing applications rely on required depends on factors including wavelength, peak high-power lasers to cut, shape, and join materials. Energy power, the nature of the material, and repetition rate. Laser transfer to the object by the laser depends on absorption pulses can even be made powerful enough to drill holes in at the laser wavelength, which varies considerably among diamond, which is among the hardest materials on Earth. materials (Figure 80). In general, metals tend to reflect more For most efficient drilling, it may be necessary to change light at longer wavelengths and absorb a larger fraction the optical focus during drilling in order to reach deeper of laser energy at shorter wavelengths. Accordingly, into the drilled hole. The laser beam and/or the object most metals are easier to cut with shorter wavelengths of can also be moved to adjust the spot being drilled. It also laser light. Titanium is a notable exception to this pattern is important to make sure that the vaporized material is because it absorbs a relatively high 8 percent of light removed from the drilling site. from a CO2 laser and is almost impossible to cut with conventional saws. Other factors also affect whether lasers LASER CUTTING or conventional tools are more appropriate for a machining Laser cutting can be thought of as drilling a series of task. For example, aluminum is rarely cut with lasers overlapping holes in a material. In cutting, the laser beam because it reflects strongly across the spectrum and is soft and/or the object move continuously, and the beam itself enough to be cut easily with ordinary saws or blades. normally operates continuously rather than being pulsed (Figure 81). Laser cutting is typically accomplished with Absorption at the solid surface does not tell the whole the assistance of a jet of air, oxygen, or dry nitrogen. For story, however. Once the material absorbs some energy, nonmetals, the role of the jet is to blow debris away from the material melts, then vaporizes. The absorption of most the cutting zone and improve the quality of the cut. Lasers materials increases with temperature, so the first bit of can cut readily through sheets of wood, paper, and plastic, heating is the hardest, and liquid metals often absorb more but thick materials are more difficult. Foods also tend to energy than the metals in the solid phase. Vaporization can char when cut, leaving an unappetizing black layer. In create problems by forming a layer of vapor or plasma other words, lasers aren’t particularly good tools for slicing between the laser and the object. This plasma layer bread—unless you prefer your toast burnt to a crisp. absorbs some laser energy, blocking it from reaching the object. Sometimes, turbulence produced by heating the Laser cutting of metals works slightly differently and uses object can remove some or all of this plasma, but in other a different type of jet. In this case, the laser beam heats cases the plasma must be removed or blown away to the metal to a temperature hot enough that it burns as the improve energy absorption by the material. FIGURE 81 The optics that focus the laser beam control how deeply the laser can penetrate the material. A lens with a short focal length focuses light into a tighter spot, but the beam spreads out faster beyond the point where the focal spot diameter Northwest Pa. Collegiate Academy - Erie, PA is smallest, reducing the concentration of light deeper in the hole. A lens with a longer focal length does not produce as tight a focal spot, but the beam does not spread out as fast, so it can drill deeper holes and cut thicker sheets of material. Machinists must take depth of focus into account when selecting the appropriate lens for the drilling or cutting task. LASER DRILLING Laser drilling is accomplished with one or more short laser pulses, each of which removes some material. The drilling process requires high peak power and works well A four-kilowatt industrial laser cutting a sheet of metal. for a sequence of short repetitive pulses. (If the energy is delivered over too long an interval in a long pulse, the © Metaveld BV / Wikimedia Commons / CC-BY-SA-3.0

2018–2019 Science78 Resource Guide oxygen from the jet passes over it. This process is properly called “laser-assisted cutting” because the oxygen in the FIGURE 82 jet is actually responsible for the combustion process that causes the cut.

Carbon dioxide lasers are commonly employed for laser cutting applications. The primary advantages of laser cutting over mechanical cutting, such as with a saw, blade, or drill, are speed and accuracy. Laser cutters can be precisely controlled, and in many cases, automated. The main disadvantage of laser cutting is the relatively high power consumption of industrial lasers. In recent years, laser cutting technology has become more accessible and affordable, making it more widely available to hobbyists. LASER BEAM WELDING Welding is effectively the opposite of cutting; cutting separates one object into two or more pieces, whereas Laser beam welding process. The laser beam is welding joins two or more pieces together into a single invisible and coming directly from above. whole (Figure 82). Both processes require the entire © Krorc / Wikimedia Commons / CC-BY-SA-3.0 thickness of the material to be heated, but the specific conditions differ. cases. Laser beam welding is commonly employed in high- In order to form a solid bond, the objects to be welded volume applications that involve automation, such as in must be fitted together precisely, leaving very little room the automotive industry. Both CO2 and Nd:YAG lasers are between them. The laser beam then heats the edges of commonly used for welding purposes. the two plates to their melting points, causing them to fuse together at the points where they are in contact. If the LASER MARKING, ENGRAVING, AND pieces are not in contact along the entire junction, the weld ETCHING will be flawed. The laser beam must also penetrate the In addition to drilling or cutting completely through entire depth of the weld (i.e., the thickness of the material) materials, lasers can be used to leave a permanent mark to form a proper joint. The heat of welding transforms the on a material, either by oxidizing the surface, carving out metal through the entire depth of the weld and in a zone a shallow indentation, or melting the outer layer. Laser extending on both sides of the actual junction. The width of marking is achieved by using a low-power laser beam to the weld zone depends on the composition of the materials heat and oxidize under the surface of the material, leaving being welded, the power delivered by the laser, and the a dark discoloration. This process leaves the surface intact speed with which the laser beam moves along the joint. but creates permanent high-contrast marks. Northwest Pa. Collegiate Academy - Erie, PA The farther from the mid-point of the weld, the less the heat affects the metal. Laser engraving uses a higher-power laser to effectively “drill” a shallow indentation in the outer layer of the material Machinists must consider several factors before welding, by vaporizing a tiny amount of surface material. As with the including the composition of the materials and their laser drilling process, several laser passes may be used to thicknesses. The materials do not have to be identical, but create deeper marks. Laser engraving is commonly used to metallurgists have learned that some types of materials do permanently mark serial numbers or logos on products or not bond together well, no matter how thoroughly they are electronic components (Figure 83). Laser etching is a subset heated or what technique is used for welding. Like cutting of laser engraving in which heat from the laser beam melts and drilling, laser welding is also limited by the thickness the surface of the material, causing it to expand slightly and of material it can handle. However, it has gained wide leaving a raised mark. Wood, plastics, metals, stone, and acceptance in manufacturing a wide variety of products in a glass can all be etched using lasers. range of sizes, including razor blades and heart pacemaker

2018–2019 Science79 Resource Guide FIGURE 83 FIGURE 84

An electronic component that has been marked by a laser.

PHOTOLITHOGRAPHY Digital devices we use every day, including smart phones, computers, and tablets, all rely on integrated circuits, or microchips, in order to function. These integrated circuits A basic illustration of the photolithography process. are made up of patterned layers of semiconductor © May Iam / Wikimedia Commons / CC-BY-SA-4.0 material and are fabricated in a laser process known as photolithography. The first step in the photolithography process is to coat the surface of a semiconductor wafer can expose features only a fraction of a wavelength. with a light-sensitive material called a photoresist. Next, an Photolithography techniques have been improved by ultraviolet light source illuminates the photoresist through immersing the optics in liquids with high refractive index, a pattern called a mask, thereby exposing patterns in the in a process known as immersion lithography. This has the photoresist. Depending on the type of photoresist, either the effect of increasing the focusing power of the lens to make exposed or unexposed areas are etched away chemically. tighter focal spots. Using a 193 nm ArF excimer laser and If additional layers of circuit are needed, another layer of liquid immersion techniques, fabricators have succeeded photoresist is deposited, and the process is repeated to at printing features smaller than 50 nm. Other efforts are

focused on improving the optical systems to stretch ArF Northwest Pa. Collegiate Academy - Erie, PA create another layer of patterns on the wafer (Figure 84). laser technology to produce finer details. Semiconductor The driving factor in increasing the power and complexity fabrication is by far the largest single application for of integrated circuits is the number of circuit elements that excimer lasers, so improving ArF technology is an important can be integrated on a single chip, which in turn depends goal within the laser industry. on the minimum feature size that can be fabricated. In order to shrink the feature size, photolithography has moved STEREOLITHOGRAPHY to shorter and shorter wavelengths. As such, ultraviolet Lasers can also be used to “build” three-dimensional excimer lasers have become the preferred light sources for objects, using techniques very different from other types fabricating microchips. The 248-nm wavelength of krypton- of laser materials working. These techniques are among fluoride excimer lasers was used in the past, but the current several competing technologies for rapid prototyping, standard is the 193-nm wavelength of argon-fluoride (ArF) which produces three-dimensional objects in shapes excimer lasers. specified by computers. Although the parts typically are used in visualization or testing, eventually they may be The resolution of photolithography can also be enhanced made on demand for general use. by using optical systems that focus the light tightly, so it

2018–2019 Science80 Resource Guide FIGURE 85 OTHER APPLICATIONS BARCODE READERS Barcode readers, also known as barcode scanners, are a familiar application of laser technology. Barcode readers work by scanning light across a pattern of dark lines of varying thickness. The dark bars absorb the light, whereas white spaces between the bars reflect the light. A photodiode measures the intensity of reflected light and interprets it as a string of information—for example, a product code in the case of a grocery store scanner or a book identification code at a library. The first laser barcode scanner was used in 1974. Although HeNe lasers were originally employed for this purpose, they have gradually been replaced with cheaper and more compact diode lasers. LASER POINTERS Laser pointers are small, battery-powered handheld devices capable of emitting a low-power beam of laser light in the visible spectrum (Figure 86). Laser pointers are Stereolithography apparatus. typically used to highlight certain items when giving a presentation, but they have other uses as well. For example, © Materialgeeza / Wikimedia Commons / CC-BY-SA-3.0 construction companies often employ laser pointers as part of surveying equipment or leveling apparatuses. One laser technique, called stereolithography, relies Robotics systems also use laser pointers as a guidance on the fact that some liquids solidify when exposed to system to orient and direct a robot toward a target. Laser ultraviolet light. In a stereolithography system, an ultraviolet light shows at entertainment venues use laser pointers laser beam is focused onto a platform slightly below the to provide an impressive spectacle, often coupling them surface in a bath of liquid that solidifies when exposed to ultraviolet light. The computer controls scanning of the FIGURE 86 beam across the surface of the liquid, forming a thin layer of solid on the platform. Then the platform is lowered further into the liquid, and the laser beam scans the surface again, following another pattern to build the next layer of the model. After that layer is formed, the platform is

lowered again, and the cycle repeats layer by layer to Northwest Pa. Collegiate Academy - Erie, PA build a three-dimensional model (Figure 85).

A related rapid prototyping technique is selective laser sintering, in which a high-power laser beam is scanned to melt small particles in a powder that covers a small platform. Then the platform is lowered, and the process is repeated to solidify another layer. Other than its use of a powder and longer- wavelength lasers, the technique is similar to stereolithography. Although stereolithography is fast and versatile, it is relatively expensive compared to other rapid prototyping techniques. Nevertheless, stereolithography systems are commercially available to hobbyists and machinists. Three laser pointers emitting light of different wavelengths in the visible spectrum.

2018–2019 Science81 Resource Guide withsmoke machines to make the full laser beam visible due to Rayleigh scattering. FIGURE 88

Because they are commercially available, the output power of laser pointers is carefully regulated to prevent accidental eye damage resulting from misuse. According to the United States Food and Drug Administration, any laser with an output power exceeding 5 mW cannot be sold or marketed as a laser pointer. It’s important to note that even “safe” low-power laser light can cause eye damage if viewed directly for extended periods. You should always exercise caution when using a laser device of any output power. LASER PRINTERS Laser printers use a laser beam to guide ink to be placed on the paper. When you click “Print,” your computer sends a stream of data to the laser printer, typically made up of millions of bytes, or single characters that encode the information you want printed. Inside the printer, an Publicity photo of the Xerox 9700, the first commercial electronic circuit interprets this data. A rotating drum laser printer in 1977. accumulates a positive electric charge. The circuit activates a laser diode that reflects a beam off a rotating hexagonal passing along positively charged particles of ink called mirror that scans it back and forth across the rotating drum. toner. The positively charged toner particles are attracted Wherever the laser beam strikes the drum, the positive to the negatively charged regions on the drum, but repelled charge is replaced by a region of negative charge instead. by the positively charged regions. Thus, an image of the The position of the laser beam is precisely guided by the page builds up in ink on the drum. A sheet of paper is then circuit to match the printing instructions: negative charge fed from a hopper within the printer, being given a positive where the page should be black, and positive charge charge as it moves toward the drum. As the paper passes where the page should be white (Figure 87). by the drum, the negatively charged toner particles are transferred over to it, and a final heated roller fuses the An ink roller comes in contact with the rotating drum, toner to the page.

In 1969, Gary Starkweather at Xerox developed the idea for FIGURE 87 laser printing. In 1977, Xerox introduced the first commercial laser printer, the Xerox 9700. Early laser printers tended to be expensive and bulky—the Xerox 9700 was roughly the

size of four modern-day photocopiers joined together (Figure Northwest Pa. Collegiate Academy - Erie, PA 88). By the late 1970s, several major computer companies, including IBM, Hewlett-Packard, and Canon were competing to develop affordable mass-market laser printers. In May 1984, Hewlett-Packard introduced the HP LaserJet, the first desktop laser printer, at a retail price of $3,495. The Apple LaserWriter printer was introduced the following year. These printers are credited with bringing desktop publishing to the mainstream.

In the decades since their introduction, laser printers have become faster, more affordable, and are now capable of printing higher-quality images than earlier models. They are A laser printer. commonly employed for high-volume printing needs, such © Department4 / Wikimedia Commons / CC-BY-SA-4.0 as in offices and schools. Although laser printers were once only capable of black-and-white printing, color versions

2018–2019 Science82 Resource Guide FIGURE 89

Diagram showing how a laser is used to read information from a CD.

© Hoikka1 / Wikimedia Commons / CC-BY-SA-3.0 Northwest Pa. Collegiate Academy - Erie, PA are now readily available. Color laser printers work by containing the track information, a reflective aluminum making three passes using different colored toners, one for layer, and a transparent polycarbonate layer that protects each of the three colors needed to generate the full range the track information. Information on a CD or DVD is of colors for reflected images. encoded as a string of ones and zeros, which can be interpreted by a disc player as a song or movie. A bump, OPTICAL DISCS: CDs, DVDs, AND known as a “pit,” corresponds to a zero byte of information. OTHERS The depth of these pits is configured to be one-quarter of In terms of sheer numbers, the widest application of laser a wavelength of the laser light in the plastic (recall that technology is the recording and playing of information on the wavelength of light changes in a transparent material optical discs such as compact discs (CDs) or digital versatile compared to air). Therefore, when the beam reflects discs (DVDs). Let’s take a closer look at how this works. back from the surface of the pit, it will be shifted a half- wavelength compared to the rest of the wave and undergo A standard CD is a disc of metal and plastic with a 4.7- destructive interference. The photodetector then interprets inch diameter. A CD consists of three layers: a plastic base the corresponding low light intensity as a zero (Figure 89).

2018–2019 Science83 Resource Guide FIGURE 90

A comparison of the laser wavelength and storage capacity of CDs, DVDs, HD DVDs, and Blu-ray. Dimensions indicated are track separation (p), pit width (w) and minimum length (l), and laser spot size ( ) and wavelength ( ).

The precursor to the CD, the LaserDisc, was introduced in the violet end of the visible spectrum). Accordingly, these 1978. A standard LaserDisc was a 12-inch-diameter metal formats have about ten times the storage capacity of a and plastic disc that had the appearance of an enlarged DVD, and 300 times that of a CD. CD. The LaserDisc format was introduced as a higher quality alternative to VHS videotape recordings. Ultimately, the high HOLOGRAPHY costs of players and discs, combined with their inability to Holography is a method of producing a three-dimensional record, led to the commercial failure of the technology. In recording of an object. The hologram itself is not an image, Northwest Pa. Collegiate Academy - Erie, PA 1995, the DVD format was introduced by Panasonic, Philips, and in many cases, it cannot be viewed under normal lighting Sony, and Toshiba. With the advent of digital media storage conditions. Holograms require laser light for both recording and internet file distribution in recent years, CDs have and viewing the hologram. The coherent nature of laser light declined in popularity and sales. It may not be long until CDs is essential to the formation of holograms. Holograms record join LaserDisc as an obsolete media format. light scattered from multiple directions rather than a single direction, as in a photograph. As such, the hologram can be A standard CD player uses an infrared laser with viewed from multiple angles and has apparent depth (Figure a wavelength of 780 nm. DVD players use shorter 91). Holography should not be confused with lenticular 3D wavelength lasers around 650 nm, which allows the pit imagery, which interlaces images from multiple angles to depth, track separation, and pit length to be scaled down create an illusion of depth, or stereoscopic imagery as seen accordingly (Figure 90). For this reason, a standard DVD in 3D films. can store approximately thirty times more information than a CD. More recent technology, such as HD DVD and Blu- A hologram is formed by splitting a laser beam into two ray discs, use even shorter wavelength lasers (405 nm, at separate beams, one of which illuminates and scatters off the object to be recorded (Figure 92). A mirror is used to

2018–2019 Science84 Resource Guide FIGURE 91

Two photographs of a hologram of a mouse, taken from two different angles.

FIGURE 92 Northwest Pa. Collegiate Academy - Erie, PA

Hologram recording process.

2018–2019 Science85 Resource Guide direct the other beam to illuminate a photographic plate ±± A related confinement technique, laser cooling, that will record the hologram. When the two laser beams uses specially configured lasers to selectively slow reach the photographic plate, the resulting interference down a collection of atoms. In 2015, a Stanford pattern is encoded on the plate. This pattern appears research lab reported cooling a rubidium gas to random. One cannot determine the image encoded on a 50 trillionths of a Kelvin. hologram simply by looking at the plate, just as one cannot ±± Lasers have been applied to nuclear fusion identify what music is recorded on a CD by looking at its research in a process known as inertial surface. The hologram can be viewed by shining a laser confinement fusion. At the National Ignition beam—identical to the one used to record it—onto the Facility, tremendously high-power lasers compress photographic film and viewing the diffraction pattern. a deuterium-tritium pellet to very high pressures Recent advances in holography methods have resulted in and temperatures for a fleeting instant. holograms that can be viewed under normal white light ±± The most commonly advertised type of laser without the aid of a laser. The most common version of surgery today, LASIK, uses an argon-fluoride these holograms is called a rainbow hologram because excimer laser to reshape the lens of the eye to it reflects a rainbow-like spectrum of colors that shifts correct refractive defects. Lasers have also been depending on the viewing angle. Rainbow holograms widely adopted for cosmetic skin treatments to are often included as a security feature on credit cards, remove wrinkles, improve skin tone, and remove passports, and even international currency. Hungarian- unwanted hair. British physicist Dennis Gabor was awarded the 1971 ±± Many industrial and manufacturing applications Nobel Prize in Physics for the invention of holography. rely on high-power lasers to cut, shape, and join Gabor devised the method of holography in the late materials. The most appropriate type of laser 1940s, years before lasers had been invented. depends on the particular machining task as well as the composition and thickness of the material. SECTION IV SUMMARY ±± Photolithography is a fabrication technique for integrated circuits, which are found in most digital Lasers are used for a variety of applications in ±± devices. Ultraviolet excimer lasers are the current scientific, medical, industrial, and commercial standard for photolithography. fields. The output power and wavelength of a laser determines the kind of applications for which it is ±± Laser printers rely on a precisely controlled laser to most appropriate. “draw” places on a rotating drum for electrically charged toner to land. The toner is then transferred Laser spectroscopy is an analysis technique in ±± to sheets of paper that are fed past the drum. which a laser is used to illuminate the sample being studied in order to determine a precise absorption ±± A familiar application of laser light is the playing spectrum. Laser absorption spectroscopy is most of CDs and DVDs. A laser beam of a specific

often performed using tunable lasers, which can frequency is used to illuminate “bumps” on an Northwest Pa. Collegiate Academy - Erie, PA be “swept” across a particular range of emitted internal layer of the disc, which encodes the wavelengths. information on the disc as a string of bytes. ±± Optical tweezers are scientific instruments that ±± Holography is a method of producing a three- use a tightly focused beam of laser light to hold dimensional recording of an object using laser microscopic particles stable in three dimensions. light. A hologram is formed by creating an Optical tweezers have been used to sort cells and interference pattern from scattered light on a measure the elasticity of DNA molecules. photographic plate, which can be read by a laser that is identical to the one used to record it.

2018–2019 Science86 Resource Guide Conclusion

Over the course of this guide, we have worked to develop an as well as to predict how images will be formed by lenses understanding of how lasers operate and what makes them and mirrors. We also found that other phenomena such such powerful tools across a wide range of applications. as diffraction, polarization, and interference can only be We saw how science’s understanding of light and other explained by considering the wave nature of light. electromagnetic waves has evolved over the course of several centuries, to the point that we are now able to In Section III, we discussed the properties and history of harness and amplify light emitted by individual atoms into the laser. We started by considering how light is emitted powerful beams of radiation for our own purposes. Although by excited atoms, and then we described various forms of there are many different categories of lasers—gas, solid- light emission, including incandescence, fluorescence, and state, semiconductor diode, and excimer—they all rely on the phosphorescence. We found that lasers can be modeled principle of stimulated emission. The wavelength and output using a combination of these emission mechanisms, power of a laser determine the types of applications for and we discussed the essential properties of laser light: which that laser is most suitable. Our understanding of how monochromaticity, directionality, and coherence. lasers interact with matter has continuously grown in the half The final section of the resource guide gave an overview of century since they were originally invented, leading to new some of the most significant applications of lasers. While applications with each passing year. lasers have served as an important research tool since their In Section I of the resource guide, we introduced light as a invention, they have also come into their own as powerful type of electromagnetic wave. We discussed the properties medical tools and versatile instruments for precision of light and considered how visible light represents the same machining. Now that you’ve reached the end of the guide, electromagnetic phenomenon as radio waves and X-rays. we hope you’ll have a deeper appreciation for the physics at work the next time you listen to a CD, watch a Blu-ray Section II covered ray and wave optics. We discussed disc, or use a laser pointer. how a ray model of light can be used to understand how light changes direction due to reflection and refraction, Northwest Pa. Collegiate Academy - Erie, PA

2018–2019 Science87 Resource Guide TIMELINE

1676 – Ole Römer makes the first qualitative measurement of the speed of light.

1690 – Christiaan Huygens publishes his wave theory of light.

1862 – Leon Foucault makes an improved measurement of the speed of light.

1865 – James Maxwell publishes a set of equations describing electric and magnetic fields.

1868 – Helium is discovered as a new element using evidence from solar spectral lines.

1887 – Heinrich Hertz produces radio waves in a laboratory.

1887 – Heinrich Hertz discovers the photoelectric effect.

1896 – Guglielmo Marconi demonstrates the use of radio waves for communication.

1900 – Max Planck suggests energy is quantized in discrete units, marking the beginning of quantum theory.

1902 – Philipp Lenard makes observations of the photoelectric effect that contradict classical theory.

1905 – Albert Einstein explains the photoelectric effect by theorizing that light is quantized.

1913 – Niels Bohr proposes a quantized model of the atom.

1917 – Albert Einstein proposes the concept of stimulated emission. Northwest Pa. Collegiate Academy - Erie, PA

1938 – E. H. Land invents the Polaroid.

1950 – Alfred Kastler proposes the technique of optical pumping.

1951 – Charles Townes devises the idea for the ammonia maser.

1955 – The first cesium atomic clock is constructed.

1957 – Gordon Gould coins the term “laser” in a scientific notebook entry.

2018–2019 Science88 Resource Guide 1960 – Theodore Maiman demonstrates the first functional laser, using ruby crystal.

1960 – The first gas laser, the helium-neon, is demonstrated.

1962 – Robert Hall demonstrates the first semiconductor diode laser.

1964 – The Nobel Prize in Physics is awarded to Charles Townes, Nikolay Basov, and Aleksandr Prokhorov.

1969 – Gary Starkweather develops the idea for laser printing at Xerox.

1969 – Confocal laser scanning microscopy is first demonstrated.

1970 – Arthur Ashkin first reports on the ability of laser light to exert forces on microscopic particles.

1974 – The first product is scanned using a laser barcode reader.

1977 – Xerox introduces the first commercial laser printer, the Xerox 9700.

1978 – LaserDisc media format is introduced.

1982 – Compact discs (CDs) become commercially available.

1984 – Hewlett-Packard introduces the first desktop laser printer, the HP LaserJet.

1986 – Arthur Ashkin and colleagues publish an article on the optical tweezing effect. Northwest Pa. Collegiate Academy - Erie, PA

1995 – DVD (Digital video disc) format is introduced.

1997 – Construction begins on the National Ignition Facility (NIF).

1999 – The LASIK refractive surgery procedure is approved in the United States.

2009 – The National Ignition Facility is completed.

The National Ignition Facility achieves the milestone of extracting more energy from a fusion reaction 2013 – than was used to trigger it.

2018–2019 Science89 Resource Guide Glossary aberration – a distortion of an image caused by confocal laser scanning microscopy (CLSM) – a unintended lens or mirror behavior technique that uses a laser to reconstruct a three- dimensional rendition of a specimen by scanning back absorption spectrum – a continuous spectrum and forth across two-dimensional slices and detecting interrupted by dark bands, observed when light passing light emitted through fluorescence through a substance is absorbed at specific frequencies constructive interference – interference between two active medium – the source of atoms that undergo waves that are in phase, meaning that their crests and excitation in a laser; can be a solid, liquid, or gas troughs align perfectly, resulting in a wave with an amplitude – the maximum displacement from the overall larger amplitude equilibrium position of a wave converging lens – a lens that refracts incoming parallel atomic number – a characteristic number for each light rays so that they meet, or converge, at a single point element, equal to the number of protons in the nucleus of convex mirror – a mirror shaped such that its reflective an atom of that element surface curves outward toward the light source; convex atomic spectroscopy – the process of analyzing spectral mirrors cannot focus light but can create images lines to identify the chemical makeup of an excited critical angle – an angle of incidence above which no sample light is refracted and all light is reflected back into the atoms – the building blocks of the world around us, made material up of a positively charged nucleus surrounded by destructive interference – interference between two electrons waves that are completely out of phase, resulting in the Brewster’s angle – see polarizing angle waves canceling each other out coherent – describes waves that are in-phase; that is, the diffraction – the bending of a wave around an edge of a

peaks of each wave are aligned with one another surface or through an aperture Northwest Pa. Collegiate Academy - Erie, PA collimated – describes a beam of light with very little diffraction grating – an optical component consisting divergence, or spread of a large number (often thousands per centimeter) of equally spaced parallel slits, which is used to produce complementary color – the color that remains when interference effects another color is subtracted from white light; for example, the complementary color of red is cyan diffuse reflection – reflection of light from a rough surface, causing reflected rays to be scattered in compound microscope – an optical instrument that uses different directions a combination of two lenses to form an enlarged virtual image of a small object diode – an electronic device that allows electric charge to pass through it in only one direction concave mirror – a mirror shaped such that its reflective surface curves inward, away from incident light; concave dispersion – the separation of light into colors due to the mirrors reflect incoming parallel light rays inward toward dependence of refractive index on wavelength a focal point diverging lens – a lens that causes light rays to spread

2018–2019 Science90 Resource Guide out, or diverge Huygens’ principle – states that every point of a wavefront can be treated as itself a source of secondary Doppler effect – a perceived change in frequency of a wavelets that spread out in all directions wave due to relative motion between the source and receiver image – the likeness of an object, formed by light rays that leave that object and are reflected by a mirror or double slit interference – a phenomenon in which light refracted by a lens passing through two thin parallel slits produces a pattern of bright and dark fringes on a screen incandescence – the production of light due to an object being at a high temperature electromagnetic spectrum – a classification of electromagnetic waves by frequency or wavelength index of refraction – a characteristic value of a transparent material, equal to the speed of light in a electromagnetic wave – a wave made up of electric vacuum divided by the speed of light in the material and magnetic fields that oscillate as the wave travels interference – the phenomenon of two waves interacting element – a substance composed of only one kind of atom to form a resultant wave emission spectrum – bands of light at specific frequencies, interferometry – the use of superimposed waves to make emitted by a source of electromagnetic radiation extremely fine measurements of small displacements, excitation – the process of an electron moving from a surface irregularities, or changes in refractive index lower to a higher energy level ion – an atom that has gained or lost electrons; ions have an excited state – any state of an atom that is higher in overall electric charge energy than its lowest (ground) state laser – a device that emits a beam of monochromatic, Fermat’s principle of least time – optical principle coherent light; laser is an acronym for “light amplification stating that the path that light takes between two points by stimulated emission of radiation” is the path that minimizes the travel time laser cooling – the technique of slowing atoms using fluorescence – an emission process in which an atom specially tuned laser light absorbs a photon and then de-excites by emitting a laser spectroscopy – a branch of spectroscopy in which photon with a lower frequency (i.e., energy) a laser is used to illuminate the sample being studied focal length – the distance from a lens to its focal point LASIK – a refractive surgery technique wherein the outer focal point – the point at which parallel light rays meet after layer of the eye is peeled back and the underlying layer passing through a converging lens or being reflected by reshaped using a laser; LASIK is an acronym for “laser- a concave mirror assisted in-situ keratomileusis”

frequency – the number of oscillations per unit time of a law of reflection – an optical principle stating that for a Northwest Pa. Collegiate Academy - Erie, PA wave or other repeating event reflected light ray, the angle of incidence is equal to the angle of reflection fusion ignition – the point at which a nuclear fusion reaction becomes self-sustaining lens – a carefully shaped piece of transparent material, such as plastic or glass, that refracts light rays to produce an gas-discharge lamp – a light source consisting of a glass image tube filled with a noble gas that is excited by an electric discharge between two electrodes light-emitting diode (LED) – a type of diode, consisting of a junction between two semiconductor layers, that geometric optics – the part of optics in which the ray emits light when an electric current is passed through it approximation is valid linearly polarized – describes light made up of waves ground state – the lowest possible energy state of an that are polarized along a single axis atom longitudinal wave – a wave that is made up of oscillations holography – a method of producing a three-dimensional that are parallel to the direction that the wave travels recording of an object

2018–2019 Science91 Resource Guide mechanical wave – a wave that requires a physical integrated circuits layer by layer; each layer is made up medium through which to propagate of a light-sensitive material called a photoresist that is exposed to laser light in a specific pattern metastable – describes an energy transition with a relatively long lifetime photon – a quantum of electromagnetic radiation that has energy proportional to its frequency mirage – a naturally occurring optical illusion due to the refraction of light through the atmosphere polarization – the plane in which the electric field of an electromagnetic wave oscillates as the wave propagates monochromatic – describes light that is made up of a single frequency polarizing angle – the angle at which reflected light will be completely polarized parallel to the surface of the Newton’s rings – an optical effect made up of concentric interface. bright and dark rings, caused by alternating constructive and destructive interference of light as it reflects from a Polaroid – a material invented by E. H. Land that can spherical lens and a flat piece of glass polarize light along a single axis normal – an imaginary line that is perpendicular to the population inversion – a necessary condition for laser surface of a material operation; occurs when more atoms within a system occupy a higher energy state than occupy a lower nuclear fusion – a nuclear reaction in which two light energy state nuclei combine to form a heavier nucleus principal axis – an imaginary line passing at a right angle nucleus – the dense core of an atom, made up of protons to the surface of the mirror or lens, through its center and neutrons principal rays – three light rays that are particularly useful opaque – describes a material that absorbs a particular for constructing ray diagrams because their behavior is frequency of light rather than transmitting it easy to predict using the refracting properties of lenses optical cavity – an arrangement of precisely aligned prism – a piece of glass or plastic typically cut into a parallel mirrors that sustains stimulated emission, triangular block, which can be used to separate white amplifies the laser light, and constrains the photons light into a spectrum of colors through dispersion emitted during laser operation to a narrow beam pump source – the energy source for a laser; the pump optical tweezers – scientific instruments that use a tightly source excites atoms in the active medium to a higher focused beam of laser light to hold microscopic particles energy level stable in three dimensions quantum – a discrete unit of energy optics – a branch of physics involving the properties of light and how light interacts with matter through reflection, quantum number – a number that indexes a particular

refraction, and diffraction quantum state of an atom Northwest Pa. Collegiate Academy - Erie, PA period – the amount of time that elapses between ray approximation – a simplification of light beams as consecutive oscillations of a wave or periodic rays disturbance ray diagram – a diagram showing the paths that light rays phosphorescence – the emission of light from a take when being reflected or refracted to form an image metastable state; the emission process takes place over ray tracing – the technique of determining or following a longer time scale than fluorescence due to the lower (tracing) the paths that light rays take in order to probability of de-excitation determine the position and size of an image photoelectric effect – the emission of electrons from the Rayleigh scattering – describes the scattering of light surface of a material when it is illuminated with light of by particles in the atmosphere, which results in high- certain frequencies frequency visible light, such as blue and violet, being photolithography – the process of using high-intensity scattered most strongly toward our eyes ultraviolet light, such as that from a laser, to create real image – an image formed by the convergence of

2018–2019 Science92 Resource Guide actual light rays; a real image can be projected onto a energy state surface stereolithography – a rapid prototyping technique reflecting telescope – an optical instrument that uses a in which a laser is scanned back and forth across combination of mirrors to collect and focus light, thereby the surface of a liquid as it is gradually lowered on a forming an image of a distant object platform, thereby solidifying the model layer by layer reflection – the change in direction of a light ray at the stimulated emission – photon emission that is triggered boundary between two materials such that it returns into by the oscillating electric field from a passing photon the medium in which it originated with the same frequency; the emitted photon is identical in frequency, phase, and direction to the first photon refracting telescope – an optical instrument that uses a combination of lenses to collect and focus light, thereby superposition – principle stating that when two waves forming an image of a distant object overlap, we can add their amplitudes together at each individual point to find the amplitude of the resulting refraction – the change in direction of a light ray as it is wave transmitted between transparent materials with different indices of refraction total internal reflection – an optical phenomenon in which a light ray traveling from a region of higher resonance – a phenomenon in which a periodic refractive index to a region of lower refractive index is disturbance matches the natural frequency of a resonator, completely reflected at the boundary causing its amplitude of oscillation to increase strongly transparent – describes a material that transmits a semiconductor – a material that can be made to conduct particular frequency of light rather than absorbing it electricity under some conditions but not others transverse wave – a wave that is made up of oscillations Snell’s law – an equation relating the angle of incidence that are perpendicular to the direction that the wave and angle of refraction for a refracted ray travels spectroscope – a measurement device that separates light virtual image – an image formed by light rays that do not emitted by a collection of excited atoms into component actually converge at the image’s location wavelengths, used for viewing emission spectra wave speed – the speed at which a wave propagates, specular reflection – reflection of light from a smooth equal to the frequency times the wavelength surface, resulting in reflected light rays remaining parallel to one another wave-particle duality – the observation that light exhibits properties of both particles and waves speed of light – the speed at which all electromagnetic waves propagate in a vacuum, equal to 299,792,458 wavelength – the distance between successive points on m/s a wave (e.g., the distance between consecutive peaks Northwest Pa. Collegiate Academy - Erie, PA spontaneous emission – the random emission of a photon due to an atomic electron moving to a lower

2018–2019 Science93 Resource Guide Notes

1. This value is exact because the length of the meter is defined usingc . 10. https://www.exploratorium.edu/snacks/cd-spectroscope. 2. Dennis F. Vanderwerf, The Story of Light Science: From Early Theories to 11. M. Egan, “Say Goodbye, Say Hello: GE Stops Making CFLs, Says Go, Today’s Extraordinary Applications. (Springer International Publishing, Go, Go To LEDs.” Feb. 1, 2016. h t t p : //g e . c o m . 2017) 4. 12. C. H. Townes, How the Laser Happened (1999), p. 69. 3. Vanderwerf 6. 13. Nobel Prize in Physics 1964, http://nobelprize.org. 4. Vanderwerf 10. 14. http://olympus.magnet.fsu.edu/primer/java/confocalsimulator/index. 5. Maxwell, J. C. “A Dynamical Theory of the Electromagnetic Field.” 1865. html. 6. Vanderwerf 11. 15. T. Kovachy, et al., “Matter Wave Lensing to Picokelvin Temperatures,” Phys. Rev. Lett. 114, 143004 (2015). 7. http://earthsky.org/earth/can-you-ever-see-the-whole-circle-of-a- rainbow. 16. “NIST Launches a New U.S. Time Standard: NIST-F2 Atomic Clock” April 3, 2014. http://www.nist.gov/pml/div688/nist-f2-atomic- 8. S.J. Schechner (2005). "Between knowing and doing: Mirrors and their clock-040314.cfm. imperfections in the Renaissance." Early Science and Medicine 10(2), 137–162. 17. Philip Ball, “Laser fusion experiment extracts net energy from fuel” Feburary 12, 2014. http://www.nature.com/news/laser-fusion- 9. A. Weis and T. L. Dimitrova. “Wave-particle duality of light for the experiment-extracts-net-energy-from-fuel-1.14710. classroom.” http://www.sps.ch/en/articles/progresses/wave-particle- duality-of-light-for-the-classroom-13/. 18. “Looking Back: The History of Laser Vision Correction” July 11, 2013. https://www.lasik.com/articles/laser-vision-correction-history/. Northwest Pa. Collegiate Academy - Erie, PA

2018–2019 Science94 Resource Guide Bibliography

“Bright Idea: The First Lasers.” American Institute of Physics, Rose, Melinda. “A History of the Laser: A Trip Through the 2017. https://history.aip.org/exhibits/laser/. Light Fantastic.” Photonics Spectra (online), May 2010. https://www.photonics.com/Article.aspx?AID=42279 Bromberg, Joan Lisa. The Laser in America, 1950–1970. The MIT Press, 1993. Serway, Raymond A. and Faughn, Jerry S. Holt Physics. Holt, Rinehart, and Winston, 2006. Hecht, Jeff. Beam: The Race to Make the Laser. Oxford University Press, 2005. Serway, Raymond, et al. College Physics. 8th ed. Cengage Learning, 2009. Hecht, Jeff. Understanding Lasers: An Entry-Level Guide. 3rd ed., John Wiley & Sons, 2008. Townes, Charles H. How the Laser Happened. Oxford University Press, 1999. Hecht, Jeff and Teresi, Dick. Laser: Light of a Million Uses, Dover Publications, 1998. Vanderwerf, Dennis F. The Story of Light Science: From Early Theories to Today’s Extraordinary Applications. Hewitt, Paul G. Conceptual Physics, 12th ed. Pearson, 2015. Springer International Publishing, 2017. Hitz, C. Breck, et al. Introduction to Laser Technology. 4th ed. John Wiley & Sons, 2012. Northwest Pa. Collegiate Academy - Erie, PA

2018–2019 Science95 Resource Guide